Add Target utility referringModule

This adds a utility method, referringModule, to the Target object that
behaves like IsMember.pathlessTarget except that it returns the module
of an InstanceTarget. This is useful for situations where you want to
get at "the module" a target is pointing at, but you want behavior
to get an actual module from an instance.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>