builder.py: check is_package_module for v2 support (#50298)

lib/spack/spack/builder.py

@@ -59,7 +59,7 @@ def __call__(self, spec, prefix):
 def get_builder_class(pkg, name: str) -> Optional[Type["Builder"]]:
     """Return the builder class if a package module defines it."""
     cls = getattr(pkg.module, name, None)
-    if cls and cls.__module__.startswith(spack.repo.PKG_MODULE_PREFIX_V1):
+    if cls and spack.repo.is_package_module(cls.__module__):
         return cls
     return None