This means import data doesn't necessarily have to have an import_url
anymore.
The `ProjectImportData` could just contain the override data in it's
serialized data attribute.
The import data is automatically cleaned up after it is finished by
the state machine.
Validate project path in Gitlab import - 10.3 port
See merge request gitlab/gitlabhq!2268
(cherry picked from commit 94c82376d66fc80d46dd2d5eeb5bade408ec6a7e)
2b94a7c2 Validate project path in Gitlab import