gh-130094: Fix race conditions in `importlib` (gh-130101)

Entries may be added or removed from `sys.meta_path` concurrently. For
example, setuptools temporarily adds and removes the `distutils` finder from
the beginning of the list. The local copy ensures that we don't skip over any
entries.

Some packages modify `sys.modules` during import. For example, `collections`
inserts the entry for `collections.abc`  into `sys.modules` during import. We
need to ensure that we re-check `sys.modules` *after* the parent module is
fully initialized.