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/hypre/package.py

@@ -62,8 +62,8 @@ def install(self, spec, prefix):
         os.environ['F77'] = spec['mpi'].mpif77
 
         # Note: --with-(lapack|blas)_libs= needs space separated list of names
-        lapack = spec['lapack'].lapack_libs
-        blas = spec['blas'].blas_libs
+        lapack = spec['lapack'].libs
+        blas = spec['blas'].libs
 
         configure_args = [
             '--prefix=%s' % prefix,