Rudimentary multi-pkg support for homebrew

This got left out when implementing multi-pkg support. This commit adds
support for "pkgs" but not "sources". I'm unable to test it currently
but would like to improve this in the future. But as it is, this commit
will prevent a few tracebacks in pkg states for Mac users, and that's a
good first step.
Merge pull request #3165 from archtaku/brew_multipkg

Rudimentary multi-pkg support for homebrew