[FR]: Provide a tar toolchain

[alexeagle]

What is the current behavior?

rules_pkg has a bunch of issues with creating tar files that include binaries:

• Fix and document pkg_tar() runfiles support. bazelbuild/rules_pkg#153
• Packaging binaries with runfiles tree bazelbuild/rules_pkg#579
• rules_pkg outputs should be usable as runfiles bazelbuild/rules_pkg#161
• pkg_tar nonfunctional with py_library bazelbuild/rules_pkg#703
• Use cases for packaging runfiles bazelbuild/rules_pkg#642
• pkg_tar for Python puts runfiles in wrong location bazelbuild/rules_pkg#715

It would be great to have rules_pkg solve these, but many are old.
rules_oci users keep running into this because rules_docker did its own runfiles handling, which rules_oci "fixed the layering violation" by expecting the pkg_tar to create a correct output.

Describe the feature

Instead of waiting for rules_pkg, we could build an alternative. It should be simple:

• We already have https://docs.aspect.build/rules/aspect_bazel_lib/docs/copy_to_directory/ - we can fix this to have correct handling of symlinks and runfiles. As a result, there should be a folder in bazel-out that looks exactly like what we want in the tar file.
• Add a toolchain that provides a tar program with predictable behavior and arguments - in practice any GNU tar that's "recent enough" works. main...tar is a start.

[alexeagle]

This is landed now and will be in the 2.0 release