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/abinit/package.py

@@ -127,13 +127,13 @@ def install(self, spec, prefix):
         # BLAS/LAPACK
         if '+scalapack' in spec:
             oapp("--with-linalg-flavor=custom+scalapack")
-            linalg = (spec['scalapack'].scalapack_libs +
-                      spec['lapack'].lapack_libs + spec['blas'].blas_libs)
+            linalg = (spec['scalapack'].libs +
+                      spec['lapack'].libs + spec['blas'].libs)
 
         # elif '+elpa' in spec:
         else:
             oapp("--with-linalg-flavor=custom")
-            linalg = spec['lapack'].lapack_libs + spec['blas'].blas_libs
+            linalg = spec['lapack'].libs + spec['blas'].libs
 
         oapp("--with-linalg-libs=%s" % linalg.ld_flags)