Make sure the CORE package is always called CORE

Make sure the CORE package is always called CORE

And not ::CORE or main::CORE or *CORE, etc.

Since the CORE package’s magic for autovivifying CORE subs
will be based on the package name, we can’t have code like
&::CORE::foo breaking the entire package.

Making a more general change to gv_fetchpvn_flags causes
problems for autoloaded SUPER methods and strangely-named
packages like main::::foo, so this patch is CORE-specific.