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/netlib-lapack/package.py

@@ -113,7 +113,7 @@ def install_one(self, spec, prefix, shared):
         if '+external-blas' in spec:
             cmake_args.extend([
                 '-DUSE_OPTIMIZED_BLAS:BOOL=ON',
-                '-DBLAS_LIBRARIES:PATH=%s' % spec['blas'].blas_libs.joined(';')
+                '-DBLAS_LIBRARIES:PATH=%s' % spec['blas'].libs.joined(';')
             ])
 
         cmake_args.extend(std_cmake_args)