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-scalapack/package.py

@@ -74,8 +74,8 @@ def install(self, spec, prefix):
         ]
 
         # Make sure we use Spack's Lapack:
-        blas = spec['blas'].blas_libs
-        lapack = spec['lapack'].lapack_libs
+        blas = spec['blas'].libs
+        lapack = spec['lapack'].libs
         options.extend([
             '-DLAPACK_FOUND=true',
             '-DLAPACK_INCLUDE_DIRS=%s' % spec['lapack'].prefix.include,