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/nwchem/package.py

@@ -73,9 +73,9 @@ class Nwchem(Package):
             patch(url, when=condition, level=0, md5=md5)
 
     def install(self, spec, prefix):
-        scalapack = spec['scalapack'].scalapack_libs
-        lapack = spec['lapack'].lapack_libs
-        blas = spec['blas'].blas_libs
+        scalapack = spec['scalapack'].libs
+        lapack = spec['lapack'].libs
+        blas = spec['blas'].libs
         # see http://www.nwchem-sw.org/index.php/Compiling_NWChem
         args = []
         args.extend([