add pacdef support (declarative package manager for Arch Linux)

[arctic-penguin]

This simple commit adds support for pacdef, which is a declarative package manager for Arch Linux.

https://github.com/steven-omaha/pacdef

Standards checklist:

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

[arctic-penguin]

Forgot to correct some variable names.