[#956] Add pkgfile support

[cleonyc]

Standards checklist:

• The PR title is descriptive.
• The code compiles (cargo build)
• The code passes rustfmt (cargo fmt)
• The code passes clippy (cargo clippy)
• The code passes tests (cargo test)
• Optional: I have tested the code myself
  • I also tested that Topgrade skips the step where needed

Closes #956
Let me know if the code is up to standards, and if I should make any changes.

Thanks,
Cleo (she/her)