[BUG] Codeowner files with Giltlab nested teams not parsed properly

[yuliazarkh]

Example of codeowner file:

* @group-x/subgroup-y/subgroup-z

throws invalid owner format '@group-x/subgroup-y/subgroup-z' at position 3

Gitlab docs: https://docs.gitlab.com/ee/user/project/codeowners/#add-a-group-as-a-code-owner