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/mfem/package.py

@@ -102,7 +102,7 @@ def install(self, spec, prefix):
         options = ['PREFIX=%s' % prefix]
 
         if '+lapack' in spec:
-            lapack_lib = (spec['lapack'].lapack_libs + spec['blas'].blas_libs).ld_flags  # NOQA: ignore=E501
+            lapack_lib = (spec['lapack'].libs + spec['blas'].libs).ld_flags  # NOQA: ignore=E501
             options.extend([
                 'MFEM_USE_LAPACK=YES',
                 'LAPACK_OPT=-I%s' % spec['lapack'].prefix.include,