Force reference consistency between Spec & Package

The correct place to set the mutual references between spec and
package objects is at the end of concretization. After a call to
concretize we should now be ensured that spec is the same object
as spec.package.spec.

Code in `build_environment.py` that was performing the same
operation has been turned into an assertion to be defensive on
the new behavior.

lib/spack/spack/build_environment.py

@@ -465,12 +465,8 @@ def setup_package(pkg, dirty):
     # code ensures that all packages in the DAG have pieces of the
     # same spec object at build time.
     #
-    # This is safe for the build process, b/c the build process is a
-    # throwaway environment, but it is kind of dirty.
-    #
-    # TODO: Think about how to avoid this fix and do something cleaner.
     for s in pkg.spec.traverse():
-        s.package.spec = s
+        assert s.package.spec is s
 
     # Trap spack-tracked compiler flags as appropriate.
     # Must be before set_compiler_environment_variables

lib/spack/spack/spec.py

@@ -1791,6 +1791,15 @@ def concretize(self):
         Concretizing ensures that it is self-consistent and that it's
         consistent with requirements of its pacakges. See flatten() and
         normalize() for more details on this.
+
+        It also ensures that:
+
+        .. code-block:: python
+
+            for x in self.traverse():
+                assert x.package.spec == x
+
+        which may not be true *during* the concretization step.
         """
         if not self.name:
             raise SpecError("Attempting to concretize anonymous spec")
@@ -1844,6 +1853,11 @@ def concretize(self):
         if matches:
             raise ConflictsInSpecError(self, matches)
 
+        # At this point the spec-package mutual references should
+        # be self-consistent
+        for x in self.traverse():
+            x.package.spec = x
+
     def _mark_concrete(self, value=True):
         """Mark this spec and its dependencies as concrete.