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/petsc/package.py

@@ -161,7 +161,7 @@ def install(self, spec, prefix):
         ])
         # Make sure we use exactly the same Blas/Lapack libraries
         # across the DAG. To that end list them explicitly
-        lapack_blas = spec['lapack'].lapack_libs + spec['blas'].blas_libs
+        lapack_blas = spec['lapack'].libs + spec['blas'].libs
         options.extend([
             '--with-blas-lapack-lib=%s' % lapack_blas.joined()
         ])