Allow PackageFilesInfo/PackageFilegroupInfo as inputs to mtree_spec() and tar()

[nnmm]

The rules_pkg package is pretty good at expressing a directory structure to be created in an archive etc., with its PackageFilesInfo and PackageFilegroupInfo providers. mtree files are similar: they also express a directory structure, except it's not known yet at analysis time.

rules_pkg could integrate nicely with the aspect_bazel_lib tar if an mtree spec could be created from targets with PackageFilesInfo or PackageFilegroupInfo. The current approach, where you'd have to mutate an mtree spec with awk or some other tool to match the package structure, is cumbersome by comparison.

Maybe as a second step, support for PackageFilesInfo/PackageFilegroupInfo could be directly added to the tar rule as well, so that the user doesn't need to separately keep the archive inputs as a flat list of files.