feat: support Debian flat repos

Fixes issue GoogleContainerTools#56

Follow-up and credit to @alexconrey (PR GoogleContainerTools#55), @ericlchen1 (PR GoogleContainerTools#64) and
@benmccown (PR GoogleContainerTools#67) for their work on similar PRs that I've reviewed and
drawn some inspiration to create "one 💍 PR to merge them all" 😅

Problem:

Debian has two types of repos: "canonical" and "flat". Each has a
different sources.list syntax:

"canonical":
```
deb uri distribution [component1] [component2] [...]
```
(see https://wiki.debian.org/DebianRepository/Format#Overview)

flat:
```
deb uri directory/
```
(see https://wiki.debian.org/DebianRepository/Format#Flat_Repository_Format)

A flat repository does not use the dists hierarchy of directories, and
instead places meta index and indices directly into the archive root (or
some part below it)

Thus, the URL logic in _fetch_package_index() is incorrect for these
repos and it always fails to fetch the Package index.

Solution:

Just use the Debian sources.list convention in the 'sources' section of
the manifest to add canonical and flat repos. Depending on whether the
channel has one directory that ends in '/' or a (dist, component, ...)
structure the _fetch_package_index and other internal logic will
know whether the source is a canonical or a flat repo.

For example:
```
version: 1

sources:
  # canonical repo
  - channel: bullseye main contrib
    url: https://snapshot-cloudflare.debian.org/archive/debian/20240210T223313Z
  # flat repos, note the trailing '/' and the lack of distribution or components
  - channel: bullseye-cran40/
    url: https://cloud.r-project.org/bin/linux/debian
  - channel: ubuntu2404/x86_64/
    url: https://developer.download.nvidia.com/compute/cuda/repos

archs:
  - amd64

packages:
  - bash
  - r-mathlib
  - nvidia-container-toolkit-base
```

Disregarding the "mixing" of Ubuntu and Debian repos for the purpose of
the example, this manifest shows that you can mix canonical and flat
repos and you can mix multiarch and single-arch flat repos and canonical
repos.

You will still have the same problems as before with packages that only
exist for one architecture and/or repos that only support one
architecture. In those cases, simply separate the repos and packages
into their own manifests.

NOTE:
The NVIDIA CUDA repos don't follow Debian specs and have issues with the
package filenames. This is addressed in a separate commit.

MODULE.bazel

@@ -70,6 +70,11 @@ apt.install(
     manifest = "//examples/debian_snapshot:bullseye_nolock.yaml",
     nolock = True,
 )
+apt.install(
+    name = "bullseye_rproject",
+    lock = "//examples/debian_flat_repo:bullseye_rproject.lock.json",
+    manifest = "//examples/debian_flat_repo:bullseye_rproject.yaml",
+)
 apt.install(
     name = "apt_security",
     manifest = "//examples/debian_snapshot_security:security.yaml",
@@ -85,4 +90,4 @@ apt.install(
     lock = "//examples/ubuntu_snapshot:noble.lock.json",
     manifest = "//examples/ubuntu_snapshot:noble.yaml",
 )
-use_repo(apt, "apt_security", "apt_security_resolve", "bullseye", "bullseye_nolock", "noble", "shared_dependencies")
+use_repo(apt, "apt_security", "apt_security_resolve", "bullseye", "bullseye_nolock", "bullseye_rproject", "noble", "shared_dependencies")

apt/private/manifest.bzl

@@ -41,8 +41,13 @@ def _source(src):
 
     index = "Packages"
 
-    index_path = "dists/{dist}/{comp}/binary-{arch}".format(**src)
-    output = "{dist}/{comp}/{arch}/{index}".format(index = index, **src)
+    if "directory" in src:  # flat repo:
+        src["directory"] = src["directory"].rstrip("/")
+        index_path = src["directory"]
+        output = "{directory}/{arch}/{index}".format(index = index, **src)
+    else:  # canonical
+        index_path = "dists/{dist}/{comp}/binary-{arch}".format(**src)
+        output = "{dist}/{comp}/{arch}/{index}".format(index = index, **src)
 
     return struct(
         arch = src["arch"],
@@ -101,15 +106,43 @@ def _from_dict(manifest, manifest_label):
 
     for arch in manifest["archs"]:
         for src in manifest["sources"]:
-            dist, components = src["channel"].split(" ", 1)
+            src["arch"] = arch
 
-            for comp in components.split(" "):
-                src["dist"] = dist
-                src["comp"] = comp
-                src["arch"] = arch
+            channel_chunks = src["channel"].split(" ")
+
+            # support both canonical and flat repos, see:
+            # canonical: https://wiki.debian.org/DebianRepository/Format#Overview
+            # flat repo: https://wiki.debian.org/DebianRepository/Format#Flat_Repository_Format
+            if len(channel_chunks) > 1:  # canonical
+                dist, components = channel_chunks[0], channel_chunks[1:]
+
+                if dist.endswith("/"):
+                    msg = "Debian dist ends in '/' but this is not a flat repo: {}"
+                    failures.append(msg.format(dist))
+
+                for comp in components:
+                    src["dist"] = dist
+                    src["comp"] = comp
+
+                    sources.append(_source(src))
+            else:  # flat
+                directory = channel_chunks[0]
+
+                if not directory.endswith("/"):
+                    msg = "Debian flat repo directory must end in '/': {}"
+                    failures.append(msg.format(directory))
+
+                src["directory"] = directory
 
                 sources.append(_source(src))
 
+    if failures:
+        for failure in failures:
+            msg = "{}: {}".format(manifest_label, failure)
+            print(msg)
+
+        fail("{}: Invalid manifest".format(manifest_label))
+
     manifest["sources"] = sources
 
     return struct(**manifest)

examples/debian_flat_repo/BUILD.bazel

@@ -0,0 +1,48 @@
+load("@container_structure_test//:defs.bzl", "container_structure_test")
+load("@rules_distroless//apt:defs.bzl", "dpkg_status")
+load("@rules_oci//oci:defs.bzl", "oci_image", "oci_load")
+
+PACKAGES = [
+    "@bullseye//dpkg",
+    "@bullseye//apt",
+    "@bullseye_rproject//r-mathlib",
+]
+
+# Creates /var/lib/dpkg/status with installed package information.
+dpkg_status(
+    name = "dpkg_status",
+    controls = [
+        "%s/amd64:control" % package
+        for package in PACKAGES
+    ],
+)
+
+oci_image(
+    name = "apt",
+    architecture = "amd64",
+    os = "linux",
+    tars = [
+        ":dpkg_status",
+    ] + [
+        "%s/amd64" % package
+        for package in PACKAGES
+    ],
+)
+
+oci_load(
+    name = "tarball",
+    image = ":apt",
+    repo_tags = [
+        "distroless/test:latest",
+    ],
+)
+
+container_structure_test(
+    name = "test",
+    configs = ["test_linux_amd64.yaml"],
+    image = ":apt",
+    target_compatible_with = [
+        "@platforms//cpu:x86_64",
+        "@platforms//os:linux",
+    ],
+)

examples/debian_flat_repo/bullseye_rproject.lock.json

@@ -0,0 +1,15 @@
+{
+	"packages": {
+		"r-mathlib": {
+			"amd64": {
+				"arch": "amd64",
+				"dependencies": [],
+				"name": "r-mathlib",
+				"sha256": "880c95a459cc2d38f4612487c6cb6e2e0d6020670f92dd951e58ba7f561425bc",
+				"url": "https://cloud.r-project.org/bin/linux/debian/bullseye-cran40/r-mathlib_4.4.2-1~bullseyecran.0_amd64.deb",
+				"version": "4.4.2-1~bullseyecran.0"
+			}
+		}
+	},
+	"version": 2
+}

examples/debian_flat_repo/bullseye_rproject.yaml

@@ -0,0 +1,20 @@
+# Packages for examples/debian_flat_repo.
+#
+#  Anytime this file is changed, the lockfile needs to be regenerated.
+#
+#  To generate the bullseye_rproject.lock.json run the following command
+#
+#     bazel run @bullseye_rproject//:lock
+#
+# See debian_package_index at WORKSPACE.bazel
+version: 1
+
+sources:
+  - channel: bullseye-cran40/
+    url: https://cloud.r-project.org/bin/linux/debian
+
+archs:
+  - amd64
+
+packages:
+  - r-mathlib

examples/debian_flat_repo/test_linux_amd64.yaml

@@ -0,0 +1,9 @@
+schemaVersion: "2.0.0"
+
+commandTests:
+  - name: "apt list --installed"
+    command: "apt"
+    args: ["list", "--installed"]
+    expectedOutput:
+      - Listing\.\.\.
+      - r-mathlib/now 4.4.2-1~bullseyecran.0 amd64 \[installed,local\]