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/dealii/package.py

@@ -145,7 +145,7 @@ def cmake_args(self):
         options = []
         cxx_flags = []
 
-        lapack_blas = spec['lapack'].lapack_libs + spec['blas'].blas_libs
+        lapack_blas = spec['lapack'].libs + spec['blas'].libs
         options.extend([
             '-DDEAL_II_COMPONENT_EXAMPLES=ON',
             '-DDEAL_II_WITH_THREADS:BOOL=ON',