Add project.group to group list when specified as owner

In the latest release, we allowed CODEOWNERS rules (https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/346) to be applied to web-based requests (such as Web IDE and file edits.) These web-requests include Merge actions ("clicking the Merge button") as well. This wider usage uncovered a bug in how we compare the user taking the action to the list of owners for a given path, specifically when the owner is the group under which the project lives (such as @gitlab-org for the Gitlab project..)

Prior to this change, Gitlab::CodeOwners::GroupsLoader did not add that group to the list of groups extracted from the incoming CODEOWNERS text and associated with the project. This list of groups is in turn expanded into a list of users that the user attempting the action (typically a merge) is compared to. Since the project's group is not added to the group list, its members are not expanded and are thus missing from the list of owners for a given path.

So, for a project such as cool-team/awesome, a path marked as owned by the @cool-team group would never be extracted from CODEOWNERS, and so it's members would fail to be recognized as having the ability to complete the merge.

This bug lead to a production incident (#216345) when codeowner rules were enabled for web requests, forcing us to do a hot-fix on 1 May.