feat: add package manager source to library

[mistydemeo]

This makes it easier to track which package manager a package comes from for non-Homebrew packages, which could be located in multiple places. I've also added a new method to get a flat list of package name strings from the linkage data.

[ashleygwilliams]

@Gankra and i discussed, want to circle up with @mistydemeo when back from vacation to get this in

[Gankra]

Whoops i rebased the wrong PR, don't mind me.

[mistydemeo]

This passes cargo test and cargo clippy, so we can get it in any time.

[ashleygwilliams]

can we make a call to get this in or close this? it's been open for a while and it'd be good to make a decision on it.

[ashleygwilliams]

@mistydemeo i think it's a really good idea so the next extra second we have, let's try to get this in!