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/elpa/package.py

@@ -59,16 +59,16 @@ def install(self, spec, prefix):
             'FC={0}'.format(self.spec['mpi'].mpifc),
             'CXX={0}'.format(self.spec['mpi'].mpicxx),
             'FCFLAGS={0}'.format(
-                spec['lapack'].lapack_libs.joined()
+                spec['lapack'].libs.joined()
             ),
             'LDFLAGS={0}'.format(
-                spec['lapack'].lapack_libs.joined()
+                spec['lapack'].libs.joined()
             ),
             'SCALAPACK_FCFLAGS={0}'.format(
-                spec['scalapack'].scalapack_libs.joined()
+                spec['scalapack'].libs.joined()
             ),
             'SCALAPACK_LDFLAGS={0}'.format(
-                spec['scalapack'].scalapack_libs.joined()
+                spec['scalapack'].libs.joined()
             ),
             '--prefix={0}'.format(self.prefix)
         ]