Add the HomeBrew Package Manager

[ripytide]

Initially requested in steven-omaha/pacdef#84

[InnocentZero]

LGTM, though I'm not certain about the brew cli args since I've never used brew.

[ajeetdsouza]

Posted this on pacdef too, but why not use brew's native bundle feature for this?

brew bundle --cleanup --no-upgrade --file=- <<EOF
brew "some-dependency-here"
EOF

[ripytide]

I saw that @ajeetdsouza but I encountered some issues with that approach.

At the moment the code is setup to work with backends which have a way to query, install and remove packages imperatively rather than declaratively which brew bundle does since we are implementing that declarativity ourselves.
A lot of the behaviour which we need to get from a backend such as brew is also not implemented as far as I can tell under the bundle subcommand such as listing missing dependencies which we need for metapac unmanaged.

Another issue I found was if we were to use brew bundle cleanup for something like metapac clean then we would need to create a brewfile which differentiates between casks and brews which resolved automatically when using brew install [packages].

Is there any advantage to using the builtin brew bundle command rather than separate brew list, brew install and brew remove calls? If not then I don't see the value in reworking the code to accommodate it.