fix(tar): expose package_dir argument in mtree_mutate

[gzm0]

This was likely forgotten in #829 when making the parameters explicit during review.

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes (stardoc)
• Breaking change (forces users to change their own code or config): no (genrule does not have a package_dir argument)
• Suggested release notes appear below: no (suggest to use commit message / PR title)

Test plan

• New test cases added

[gzm0]

Heads-up: Potentially interacts with #852. /cc @sin-ack

[sin-ack]

This shouldn't affect us except for a stray / added in some cases (which does not change the meaning of the mtree spec). Is that an issue? I can move my slash-appender as a separate pass below the package_dir pass if needed.

[gzm0]

TY for checking. I doubt it's an issue, but I just wanted you to have a look at it, because it seems you have much more mtree spec experience then I do (I don't have any, I just wanted to prepend a string 🤷‍♂️ ).

After re-reading the description on #852, it indeed looks like the two changes should be independent.