multimethod uses Spec() instead of parse_anonymous_spec()

- simplify logic in multimethod
- remove the requirement of multimethod invocations to walk up the stack.

var/spack/repos/builtin.mock/packages/multimethod/package.py

@@ -21,7 +21,7 @@ class Multimethod(MultimethodBase):
     url      = 'http://www.example.com/example-1.0.tar.gz'
 
     #
-    # These functions are only valid for versions 1, 2, and 3.
+    # These functions are only valid for versions 1, 3, and 4.
     #
     @when('@1.0')
     def no_version_2(self):