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/psi4/package.py

@@ -62,10 +62,10 @@ class Psi4(Package):
     def install(self, spec, prefix):
         cmake_args = [
             '-DBLAS_TYPE={0}'.format(spec['blas'].name.upper()),
-            '-DBLAS_LIBRARIES={0}'.format(spec['blas'].blas_libs.joined()),
+            '-DBLAS_LIBRARIES={0}'.format(spec['blas'].libs.joined()),
             '-DLAPACK_TYPE={0}'.format(spec['lapack'].name.upper()),
             '-DLAPACK_LIBRARIES={0}'.format(
-                spec['lapack'].lapack_libs.joined()),
+                spec['lapack'].libs.joined()),
             '-DBOOST_INCLUDEDIR={0}'.format(spec['boost'].prefix.include),
             '-DBOOST_LIBRARYDIR={0}'.format(spec['boost'].prefix.lib),
             '-DENABLE_CHEMPS2=OFF'