Make multimethods work with inheritance. (#3411)

Previously, this would fail with a NoSuchMethodError:

    class Package(object):
        # this is the default implementation
        def some_method(self):
            ...

    class Foo(Package):
        @when('platform=cray')
        def some_method(self):
            ...

        @when('platform=linux')
        def some_method(self):
            ...

This fixes the implementation of `@when` so that the superclass method
will be invoked when no subclass method matches.

Adds tests to ensure this works, as well.

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

@@ -25,8 +25,10 @@
 from spack import *
 import spack.architecture
 
+from spack.pkg.builtin.mock.multimethod_base import MultimethodBase
 
-class Multimethod(Package):
+
+class Multimethod(MultimethodBase):
     """This package is designed for use with Spack's multimethod test.
        It has a bunch of test cases for the @when decorator that the
        test uses.
@@ -132,3 +134,11 @@ def different_by_virtual_dep(self):
     @when('^mpi@2:')
     def different_by_virtual_dep(self):
         return 2
+
+    #
+    # Make sure methods with a default implementation in a superclass
+    # will invoke that method when none in the subclass match.
+    #
+    @when("@2:")
+    def base_method(self):
+        return "subclass_method"