Stop enabling all features by default in libherokubuildpack

[edmorley]

By default libherokubuildpack has all Cargo features enabled:

libcnb.rs/libherokubuildpack/Cargo.toml

Line 21 in d2b7475

default = ["command", "download", "digest", "error", "log", "tar", "toml", "fs", "write"]

It's then up to consumers of the crate to know that they probably should disable features they aren't using, to save pulling in lots of unnecessary dependencies.

Whilst this reduces the confusion/friction for people who don't know how to enable a Cargo feature (since everything works out of the box), its very rare that anyone needs all features, so literally every consumer should really switch to default-features = false eventually.

Lots of consumers of the crate aren't doing this (even those maintained by experienced Rust users), eg:

• https://github.com/heroku/buildpacks-jvm/blob/b56ce4e69c704a0f29c9cf15a823f5b6b0824a13/buildpacks/gradle/Cargo.toml#L13
• https://github.com/heroku/buildpacks-jvm/blob/b56ce4e69c704a0f29c9cf15a823f5b6b0824a13/buildpacks/sbt/Cargo.toml#L14
• https://github.com/heroku/buildpacks-jvm/blob/b56ce4e69c704a0f29c9cf15a823f5b6b0824a13/buildpacks/jvm-function-invoker/Cargo.toml#L12
• https://github.com/heroku/buildpacks-nodejs/blob/8e224d4c36dae7f2e60de3821f4ad3f6a76609b9/buildpacks/nodejs-engine/Cargo.toml#L12
• https://github.com/heroku/buildpacks-nodejs/blob/8e224d4c36dae7f2e60de3821f4ad3f6a76609b9/buildpacks/nodejs-npm-engine/Cargo.toml#L15
• https://github.com/heroku/buildpacks-ruby/blob/4322def6c6e4c709a44bea42de8b3518cc83445c/commons/Cargo.toml#L24

And even the ones that are, often are doing so only because I fixed them:

• heroku/buildpacks-procfile@4d31dc8#diff-2e9d962a08321605940b5a657135052fbcef87b5e360662bb527c96d9a615542R12
• heroku/heroku-jvm-application-deployer@a670871
• heroku/buildpacks-php@6558c04#diff-880bd5f73568fdc2dc5a91f0d09fb5f7e93640562dd91ad67bf9a3164fef1097R19

We should either not enable any features by default, or only enable the most commonly used ones (eg logging).