'Return the specs of all packages that extend
the given spec'

@_autospec
def installed_extensions_for(self, extendee_spec):

    for spec in self.query():
        try:
            spack.store.layout.check_activated(extendee_spec, spec)
            (yield spec.package)
        except spack.directory_layout.NoSuchExtensionError:
            continue
