Allow users to import GitHub projects to subgroups

What does this MR do?

Find the selected namespace by full path.

Why was this MR needed?

User can't import projects to subgroups.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #30289 (closed)

Closes #30148 (closed)