Add explicit CMakePackage import to Spack package

- Import CMakePackage from spack.build_systems.cmake
- CMakePackage not included in 'from spack.package import *'
- Fixes 'name CMakePackage is not defined' error in CI

Author: sbryngelson

packaging/spack/package.py

@@ -4,6 +4,7 @@
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
 from spack.package import *
+from spack.build_systems.cmake import CMakePackage
 
 
 class Mfc(CMakePackage):