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 = []

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

@@ -59,7 +59,7 @@ class Ipopt(AutotoolsPackage):
 
     patch('ipopt_ppc_build.patch', when='arch=ppc64le')
 
-    flag_handler = AutotoolsPackage.build_system_flags
+    flag_handler = build_system_flags
     build_directory = 'spack-build'
 
     # IPOPT does not build correctly in parallel on OS X