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/hpl/package.py

@@ -78,7 +78,7 @@ def configure(self, spec, arch):
             'MPlib        = -L{0}'.format(spec['mpi'].prefix.lib),
             # Linear Algebra library (BLAS or VSIPL)
             'LAinc        = {0}'.format(spec['blas'].prefix.include),
-            'LAlib        = {0}'.format(spec['blas'].blas_libs.joined()),
+            'LAlib        = {0}'.format(spec['blas'].libs.joined()),
             # F77 / C interface
             'F2CDEFS      = -DAdd_ -DF77_INTEGER=int -DStringSunStyle',
             # HPL includes / libraries / specifics