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/cantera/package.py

@@ -85,7 +85,7 @@ def install(self, spec, prefix):
 
         # BLAS/LAPACK support
         if '+lapack' in spec:
-            lapack_blas = spec['lapack'].lapack_libs + spec['blas'].blas_libs
+            lapack_blas = spec['lapack'].libs + spec['blas'].libs
             options.extend([
                 'blas_lapack_libs={0}'.format(','.join(lapack_blas.names)),
                 'blas_lapack_dir={0}'.format(spec['lapack'].prefix.lib)