Merge pull request #399 from LLNL/bugfix/398-extensions-error

fix for issue 398, added extensions_for in RepoPath

lib/spack/spack/repository.py

@@ -233,6 +233,11 @@ def providers_for(self, vpkg_spec):
         return providers
 
 
+    @_autospec
+    def extensions_for(self, extendee_spec):
+        return [p for p in self.all_packages() if p.extends(extendee_spec)]
+
+
     def find_module(self, fullname, path=None):
         """Implements precedence for overlaid namespaces.