Fix when_spec inheritance for extensions (#7746)

Popping the when spec from kwargs in the extends directive breaks
class inheritance. Inheriting classes do not find their when spec.

We now get the when spec from kwargs instead, leaving it to be found
by any downstream package classes.

lib/spack/spack/directives.py

@@ -381,7 +381,7 @@ def _execute_extends(pkg):
         #     msg = 'Packages can extend at most one other package.'
         #     raise DirectiveError(directive, msg)
 
-        when = kwargs.pop('when', pkg.name)
+        when = kwargs.get('when', pkg.name)
         _depends_on(pkg, spec, when=when)
         pkg.extendees[spec] = (Spec(spec), kwargs)
     return _execute_extends