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/octave/package.py

@@ -108,8 +108,8 @@ def configure_args(self):
 
         # Required dependencies
         config_args.extend([
-            "--with-blas=%s" % spec['blas'].blas_libs.ld_flags,
-            "--with-lapack=%s" % spec['lapack'].lapack_libs.ld_flags
+            "--with-blas=%s" % spec['blas'].libs.ld_flags,
+            "--with-lapack=%s" % spec['lapack'].libs.ld_flags
         ])
 
         # Strongly recommended dependencies