Directive inheritance: laziness for the win (#2623)

* inheritance of directives: using meta-classes to inject attributes coming from directives into packages + lazy directives

* _dep_types -> dependency_types
* using a meta-class to inject directives into packages
* directives are lazy

fixes #2466

* directives.py: allows for multiple inheritance. Added blank lines as suggested by @tgamblin

* directives.py: added a test for simple inheritance of directives

* Minor improvement requested by @tgamblin

CMakePackage: importing names from spack.directives
directives: wrap __new__ to respect pep8

* Refactoring requested by @tgamblin

directives: removed global variables in favor of class variables. Simplified the interface for directives (they return a callable on a package or a list of them).

lib/spack/llnl/util/lang.py

@@ -342,38 +342,6 @@ def match(string):
     return match
 
 
-def DictWrapper(dictionary):
-    """Returns a class that wraps a dictionary and enables it to be used
-       like an object."""
-    class wrapper(object):
-
-        def __getattr__(self, name):
-            return dictionary[name]
-
-        def __setattr__(self, name, value):
-            dictionary[name] = value
-
-        def setdefault(self, *args):
-            return dictionary.setdefault(*args)
-
-        def get(self, *args):
-            return dictionary.get(*args)
-
-        def keys(self):
-            return dictionary.keys()
-
-        def values(self):
-            return dictionary.values()
-
-        def items(self):
-            return dictionary.items()
-
-        def __iter__(self):
-            return iter(dictionary)
-
-    return wrapper()
-
-
 def dedupe(sequence):
     """Yields a stable de-duplication of an hashable sequence