This is a cleanup in preparation for !7. Originally, !7 extended the use of `current_modref`, but I think we could avoid that with a more direct approach. Most of these changes are no-ops.
For the relay handler, we no longer treat `GetProcAddress` calls inside `DllMain` as coming from a loaded DLL. I believe this was more of a side effect of how the code was structured rather than an intentional behavior that needs to be preserved.
One more use of `current_modref` remains, which will be removed by !7.
-- v2: ntdll: Remove no longer needed current_modref. null: Use NULL importer in LdrGetProcedureAddress. ntdll: Pass importer pointer to build_import_name. ntdll: Pass importer pointer to find_forwarded_export. ntdll: Pass importer pointer to find_named_export. ntdll: Pass importer pointer to find_ordinal_export. ntdll: Pass WINE_MODREF to import_dll.