Support packaging raw symlinks from declare_symlink

[rickeylev]

Over in rules_python, declare_symlink() is used to create symlinks for efficiency and correctness reasons. Such artifacts are "raw" symlinks, that is, Bazel will keep them exactly as-is and not try to transform them, as happens when using declare_file().

Unfortunately, pkg_tar gives an error when it sees these artifacts. What happens is it thinks the file is a regular file, so tries to read it. Since its a relative symlink, it either is dangling (in the context of pkg_tar), or will read the actual file (instead of creating a symlink), neither of which is desirable behavior.

Thankfully, starting in Bazel 8, File.is_symlink can be used to detect these artifacts. This allows the manifest that pkg_tar generates, and the build_tar program that uses it, to handle them correctly.

The two basic changes are:

1. When File.is_symlink is true, write a manifest entry with entry_type=raw_symlink (pkg_files.bzl)
2. Modify build_tar.py to handle entry_type=raw_symlink: read os.readlink() from the src file. Then store it as a symlink. This preserves the original symlink.

I prototyped this a bit and it seems to work. I'd reference my branch, but its in a very hacky and broken state right now. The above gets the gist though

[rickeylev]

I sent out #930 to demonstrate the gist of fixes necessary for pkg_tar.

Something to note is that File.is_symlink is a Bazel 8+ API. For earlier versions of Bazel, I'm not aware of a way to detect these. For earlier Bazel versions, a workaround I thought of is to have rules_pkg check for OutputGroupInfo.files_that_are_symlinks. A separate provider could be used, too. The reason I suggest a more structural-based approach is to avoid the issue of language rules having to export multiple packaging-implementation-specific providers or vice versa.