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/wannier90/package.py

@@ -47,8 +47,8 @@ class Wannier90(Package):
 
     def install(self, spec, prefix):
 
-        lapack = self.spec['lapack'].lapack_libs
-        blas = self.spec['blas'].blas_libs
+        lapack = self.spec['lapack'].libs
+        blas = self.spec['blas'].libs
         substitutions = {
             '@F90': spack_fc,
             '@MPIF90': self.spec['mpi'].mpifc,