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

var/spack/repos/builtin/packages/py-numpy/package.py

@@ -70,10 +70,10 @@ def patch(self):
         # for build notes see http://www.scipy.org/scipylib/building/linux.html
         lapackblas = LibraryList('')
         if '+lapack' in spec:
-            lapackblas += spec['lapack'].lapack_libs
+            lapackblas += spec['lapack'].libs
 
         if '+blas' in spec:
-            lapackblas += spec['blas'].blas_libs
+            lapackblas += spec['blas'].libs
 
         if '+blas' in spec or '+lapack' in spec:
             # note that one should not use [blas_opt] and [lapack_opt], see