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/intel-mkl/package.py

@@ -83,7 +83,7 @@ def blas_libs(self):
 
     @property
     def lapack_libs(self):
-        return self.blas_libs
+        return self.libs
 
     @property
     def scalapack_libs(self):