Fix hash handling in directory layout

- Currently, build dependencies are not currently hashed; we are waiting
  to hash these until we have smarter concretization that can reuse more
  installed specs.  The layout needs to account for this when checking
  whethert things are installed.

lib/spack/spack/directory_layout.py

@@ -268,6 +268,13 @@ def check_installed(self, spec):
         if installed_spec == spec:
             return path
 
+        # DAG hashes currently do not include build dependencies.
+        #
+        # TODO: remove this when we do better concretization and don't
+        # ignore build-only deps in hashes.
+        elif installed_spec == spec.copy(deps=('link', 'run')):
+            return path
+
         if spec.dag_hash() == installed_spec.dag_hash():
             raise SpecHashCollisionError(spec, installed_spec)
         else: