Make builtin flag handlers available in package scope (#8668)

* Push default flag handlers into module scope

* Preserve backwards compatibility of builtin flag handler names

Ensure Spack continues to work for packages using the `Package.env_flags` idiom and equivalent.

* update docs and tests to match

* Update packages to match new syntax

var/spack/repos/builtin/packages/hiop/package.py

@@ -48,7 +48,7 @@ class Hiop(CMakePackage):
     depends_on('lapack')
     depends_on('blas')
 
-    flag_handler = CMakePackage.build_system_flags
+    flag_handler = build_system_flags
 
     def cmake_args(self):
         args = []