fix: make a patched test patch what actually runs

A test isolated the extra collection by patching
resolve_platform_cores on packextras with create=True, while the
module imported it inside the function, so the real resolver ran.
The import is now module level, the test checks its stub was called,
and no test may create the attribute it patches.

scripts/packextras.py

@@ -14,6 +14,7 @@ from common import declared_root, expand_platform_declared_names, platform_decla
 from common import load_emulator_profiles
 from common import resolution_is_hash_exact
 from common import resolve_local_file
+from common import resolve_platform_cores
 from common import runs_standalone
 from common import sanitize_pack_path
 from common import size_fits
@@ -616,7 +617,6 @@ def _collect_emulator_extras(
 
     Works for ANY platform (RetroArch, Batocera, Recalbox, etc.)
     """
-    from common import resolve_platform_cores
     from verify import find_undeclared_files
 
     profiles = (