Preinstalled packages

On Mac OS X, some packages are preinstalled system packages, which means
that Homebrew (what we're using as the native package manager) doesn't
know about them.  That means they end up looking like they're
uninstalled and not available.  You can now set a
`preinstalled.homebrew.[package]` variable to tell Buzzy about these
preinstalled packages.  The value of the variable should be the version
number that is preinstalled.