These will be used to unify the label namespaces in embedded plists,
i.e. 'homebrew.mxcl.<formula>'.
plist_path returns the full path to a plist file located at the top
level of the keg; plist_path.basename can be used if just the filename
is needed.
c.f. Homebrew/homebrew#9346.
Signed-off-by: Jack Nagel <jacknagel@gmail.com>