Unify behavior of source code search

[disinvite]

This started with fixing a problem observed in #331 and evolved from there.

Fixing compatibility issues in walk_source_dir

walk_source_dir iterates the root source code directory for a decomp project. #279 changed this to use Path.walk() instead of os.walk(). The problem is that Path.walk() is only available on Python 3.12 and higher, but we still support a few versions earlier than that. This was missed because no unit tests targeted walk_source_dir. We didn't hear anything from users so hopefully the explanation is that no one is using reccmp with older Python versions.

This restores the older os.walk() implementation so we can keep compatibility with Python 3.10 and 3.11. We now have unit tests to verify the behavior of walk_source_dir.

Returning source code paths in a consistent order

decomplint already supported multiple source code search paths at the command line, and we will add this option in #331. In the main reccmp script, there is the potential for data to be overwritten if we read metadata for the same address in multiple files. Therefore, we need to ensure:

1. Paths are deduplicated so we do not scan the same file twice
2. Paths are scanned in the same order across platforms

The second point is an issue because pathlib.PosixPath is sorted in case-sensitive order, but pathlib.WindowsPath is not. The paths are now always sorted in case-insensitive order so the output will be the same across platforms.

To bring this all together, we now have a source_code_search function. Tests are included.

[jonschz]

Looks good, the test coverage is nice. I didn't check for regressions in great detail, though

[disinvite]

Looks good, the test coverage is nice. I didn't check for regressions in great detail, though

The only new thing should be the ordering of paths, but I don't expect a change in behavior unless both are true:

1. The paths/files in a project aren't named consistently (i.e. not all lowercase).
2. The same address is used in an annotation across multiple files.

I checked with a few projects on Windows and didn't see a diff. No diff for LEGO1 between Mac and Windows either.