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/opium/package.py

@@ -37,7 +37,7 @@ class Opium(Package):
     depends_on('lapack')
 
     def install(self, spec, prefix):
-        libs = spec['lapack'].lapack_libs + spec['blas'].blas_libs
+        libs = spec['lapack'].libs + spec['blas'].libs
         options = ['LDFLAGS=%s' % libs.ld_flags]
 
         configure(*options)