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/elk/package.py

@@ -90,9 +90,9 @@ def configure(self, spec):
         blas = 'blas.a'
         lapack = 'lapack.a'
         if '+blas' in spec:
-            blas = spec['blas'].blas_libs.joined()
+            blas = spec['blas'].libs.joined()
         if '+lapack' in spec:
-            lapack = spec['lapack'].lapack_libs.joined()
+            lapack = spec['lapack'].libs.joined()
         # lapack must come before blas
         config['LIB_LPK'] = ' '.join([lapack, blas])