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/armadillo/package.py

@@ -55,9 +55,9 @@ def install(self, spec, prefix):
             # ARPACK support
             '-DARPACK_LIBRARY={0}'.format(arpack.joined()),
             # BLAS support
-            '-DBLAS_LIBRARY={0}'.format(spec['blas'].blas_libs.joined()),
+            '-DBLAS_LIBRARY={0}'.format(spec['blas'].libs.joined()),
             # LAPACK support
-            '-DLAPACK_LIBRARY={0}'.format(spec['lapack'].lapack_libs.joined()),
+            '-DLAPACK_LIBRARY={0}'.format(spec['lapack'].libs.joined()),
             # SuperLU support
             '-DSuperLU_INCLUDE_DIR={0}'.format(spec['superlu'].prefix.include),
             '-DSuperLU_LIBRARY={0}'.format(superlu.joined()),