docs: Avoid use of rules_pkg

[jfirebaugh]

I'd like to propose that rules_oci avoid explicit or implicit endorsement of rules from rules_pkg, particularly pkg_tar. Where documentation and examples currently refer to pkg_tar, use the tar rule from bazel-lib instead.

Rationale: in addition to the advantages of tar listed in the bazel-lib documentation, rules_pkg is poorly maintained and has surprising behaviors such as flattening directory trees and omitting runfiles by default, which seem unlikely to ever be fixed.

[thesayyn]

This sounds fine, irony is that i was the one started bazel-lib tar :) but somehow left rules_pkg usage here intact. I'd be happy to review a PR, if this is something you feel like doing.

[sfc-gh-mhazy]

This probably should be a separate issue, but wanted to leave a comment about a problem I experienced with aspect's tar.

• tar is a macro that declares two rules: mtree_spec and tar_rule
• mtree_spec produces a map of filepaths that is later consumed by tar_rule.
• Since these are 2 separate rules, there is no guarantee on where these are executed, unlike if these were 2 actions of a single rule.
• I think this caused us a cache poisoning when executing remotely. mtree_spec results assumed some file paths, which were different when executing tar_rule.
• in rules_pkg the mapping context is created as part of pkg_tar rule implementation, which avoids problems mentioned above.

Sorry for distraction.