Unable to fork a project within the same namespace via API

@spikeburton reported in gitlab-foss#40798 (comment 294739648):

I am now unable to fork a project into a namespace, and am getting a 404 response with the error:

{
    "message": "404 Target Namespace Not Found"
}

My payload looks like the following:

{
    "namespace": 6535607,
    "path": "test-fork",
    "name": "test-fork"
}

Technical details

Target namespace is the same as project namespace in report above. It should be a valid namespace if alternative path and name are specified for a fork.

Proposal

• Allow to fork to the same namespace as project namespace
• Exclude project namespace from list of possible fork targets for UI (since we don't allow to specify alternative path and name there)