Don't create new namespaces when importing via GitHub import task

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/111717



Merged-by: Gabriel Mazetto <brodock@gmail.com>
Approved-by: Rodrigo Tomonari <rtomonari@gitlab.com>
Approved-by: Gabriel Mazetto <brodock@gmail.com>
Approved-by: Evan Read <eread@gitlab.com>
Reviewed-by: Rodrigo Tomonari <rtomonari@gitlab.com>
Co-authored-by: cervols <paprikamail@gmail.com>