Tweak symlinks in create_input

[kylewlacy]

This PR fixes some issues around symlink handling in the create_input function. I found this after finding a bug introduced by #115, but also found some other issues too.

First, when calling create_input on a directory, if two files refer to the same resource path and that path is a symlink, then that resource would only get added to one of the files. This was due to some unsound memoization, which has now been removed.

Second, the way that symlink resources were canonicalized meant that, in some situations, create_input could end up creating a directory structure that didn't match the input. I worked out two such cases:

1. When a symlink's target points to another symlink (link1 -> link2, link2 -> target)
2. When a symlink's target has another symlink as a path component (link1 -> link2/file, link2 -> dir)

For now, I made the call to just return an error if either of these two situations occur. I don't believe either case was used in practice (although, if it was, hopefully brioche-packages will make that pretty clear). I believe a proper implementation to support both cases would be very complicated and error-prone, so just throwing an error now will make it possible to revisit in the future