pkg on Mac OSX: Support full package names
pkg on Mac OSX: Support full package names

This makes _verify_install ignore everything up to and including the
final slash in a package name on Mac OSX, if that is necessary to find
the correct package.

This allows specifications to be prefixed with a tap name, like
"homebrew/science/a5".

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt