feat(pkg_files): preserve external package paths by default

[rdesgroppes]

When no explicit strip_prefix is specified, pkg_files now automatically preserves package-relative paths for cross-package and cross-repository references.
This eliminates the need for manual strip_prefix configurations in these common scenarios.

Cross-repository example:
@external//pkg → @//tests:my_files (contains tests/testdata/file.txt)

strip_prefix	Result
strip_prefix.from_root("tests")	testdata/file.txt
strip_prefix.from_pkg()	testdata/file.txt
None (earlier default behavior)	file.txt
None (new default behavior)	testdata/file.txt

Cross-package example:
//pkg_a → //pkg_b:my_files (contains pkg_b/subdir/file.txt)

strip_prefix	Result
strip_prefix.from_root("pkg_b")	subdir/file.txt
strip_prefix.from_pkg()	subdir/file.txt
None (earlier default behavior)	file.txt
None (new default behavior)	subdir/file.txt

Backward compatibility: since direct cross-package target references now preserve package paths instead of basename only, referencing files in a local filegroup aggregating cross-package files would bring back basename-only paths.

[cgrindel]

LGTM. It looks like there is a pre-commit failure. Please ping us when it is addressed.

[rdesgroppes]

LGTM. It looks like there is a pre-commit failure. Please ping us when it is addressed.

Indeed, I saw https://github.com/bazelbuild/rules_pkg/actions/runs/21136051279/job/61362936971#step:3:156

Done. I think the reason why I had to add a docstring to test_referencing_remote_file is because it now has 5+ statements.

[rdesgroppes]

Had to document ǹame as well, sorry...

[cgrindel]

@aiuto Do you want to review before we merge?

[aiuto]

I don't understand the behavior diff for the same workspace, inter package thing.
Is this a potentially breaking change in any way?

[rdesgroppes]

I don't understand the behavior diff for the same workspace, inter package thing. Is this a potentially breaking change in any way?

I've done my best to exemplify the change with further tests and added the following note to the commit message/PR description:

Backward compatibility: since direct cross-package target references now preserve package paths instead of basename only, referencing files in a local filegroup aggregating cross-package files would bring back basename-only paths.

[rdesgroppes]

I think I'm going to abandon this PR because I realize there's quite a number of issues reported on the current API:

• package_dir unexpectedly being stripped from files when it matches Bazel package name #905
• Behavior of strip_prefix.files_only() and strip_prefix.from_pkg() swapped between pkg_files and pkg_tar #805
• document strip_prefix in the unified reference pages #482
• pkg_tar.strip_prefix does not work for srcs from other packages. #462
• pkg_files(strip_prefix) behavior is confusing w.r.t. flattening a tree of files #354
• mappings.bzl strip_prefix pseudomodle functions should produce unique outputs #333
• document that directories are flattened unless data_path, strip_prefix are set to "." #82

... so my change would probably lead to more confusion.

Sorry, but thanks for having taken the time to consider it.