The current subgroup migration strategy works iteratively, which means
that each level of a group tree is imported before moving to the next
level.

This becomes a problem when the target group is within the source group,
which creates a _infinity_ loop of import.

>> Example:

Given a group tree like:

- Group A
  - Group B
  - Group C
    - Group X
    - Group Y

If one decide to import the Group A into Group B, a recursion will
happen, making the import stop only when the depth limit is achieved or
some other database constraint, like the size of a group path.

- Group A
  - Group B
    - Group A
      - Group B
        - Group A
          - Group B
            - Group A
              - Group B
                - ...
              - Group C
                - Group X
                - Group Y
          - Group C
            - Group X
            - Group Y
      - Group C
        - Group X
        - Group Y
  - Group C
    - Group X
    - Group Y

To fix that, destination namespaces cannot be a descendant of the source
group.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/300445