Respect namespace_id for bitbucket server importer

What does this MR do and why?

Related to Importers - Default to the current group when i... (#359130 - closed)

This MR correctly reuses namespace_id through import process of bitbucket server importer

Screenshots or screen recordings

Flightjs___GitLab__3_

How to set up and validate locally

1. Set up new bitbucket server instance (easiest way is using docker https://hub.docker.com/r/atlassian/bitbucket )
2. Visit any group on your gitlab
3. New project -> Import -> Bitbucket server
4. Use credentials to your instance
5. Observe that group selected as default import

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.