Add package_dir method to package.py.

lib/spack/spack/package.py

@@ -397,6 +397,12 @@ def __init__(self, spec):
         if self.is_extension:
             spack.repo.get(self.extendee_spec)._check_extendable()
 
+    @property
+    def package_dir(self):
+        """Return the directory where the package.py file lives."""
+        return os.path.dirname(self.module.__file__)
+
+
     @property
     def global_license_dir(self):
         """Returns the directory where global license files for all