New interface for passing build information among specs (#1875)

- Added a new interface for Specs to pass build information
  - Calls forwarded from Spec to Package are now explicit
  - Added descriptor within Spec to manage forwarding
  - Added state in Spec to maintain query information
  - Modified a few packages (the one involved in spack install pexsi) to showcase changes

- This uses an object wrapper to `spec` to implement the `libs` sub-calls.
  - wrapper is returned from `__getitem__` only if spec is concrete
  - allows packagers to access build information easily

lib/spack/llnl/util/lang.py

@@ -374,3 +374,17 @@ def duplicate_stream(original):
     :rtype: file like object
     """
     return os.fdopen(os.dup(original.fileno()))
+
+
+class ObjectWrapper(object):
+    """Base class that wraps an object. Derived classes can add new behavior
+    while staying undercover.
+
+    This class is modeled after the stackoverflow answer:
+      -  http://stackoverflow.com/a/1445289/771663
+    """
+    def __init__(self, wrapped_object):
+        wrapped_cls = type(wrapped_object)
+        wrapped_name = wrapped_cls.__name__
+        self.__class__ = type(wrapped_name, (type(self), wrapped_cls), {})
+        self.__dict__ = wrapped_object.__dict__