feat: better path handling when using --find-links especially

[tdejager]

What does it fix

1. This should help with locking and trying to use relative paths when possible. For example before the find-links example had absolute path in the lock file, now we should not. Also compared to before the code has been documented a lot better, and there were some assumption errors, like assuming a path would be relative.

2. I've noticed that if a panic occurs during a resolve that this sometimes led to a silent error, very annoying. So I just removed all expect's from the resolve code and decided to use errors and miette for all of it.

How the design is approached

The assumption now is to use a relative path in the lock file if possible and otherwise fall back to an absolute path. Note that this behavior is only changed for path based indexes. We now check what kind of index the package is coming from, the latest uv updates made these distinctions a lot easier.

Why not always use relative paths?

Because (and some tests do this) you can supply absolute paths to indexes on disk or flat indexes (which are basically just a list of files) with absolute paths. However, if the project root is contained in this path we will make it relative, this way you can supply a relative path to a flat index, and it should work. Currently, we allow absolute paths, and like I said above some tests make absolute paths to extra-index-url on disk.

How to test

• Find links examples
• clean run of all examples
• Maybe some manual testing with both absolute and non-absolute indexes.