Ensure import by URL works after a failed import

If a project fails to import a project with one of the built-in importers (e.g. GitHub, Bitbucket, GitLab project templates, etc.), a user will be redirected to the /import/new page. The user can attempt to import with a Git repository URL, but that would also fail because import_type remained what it was set to initially (e.g. gitlab_project). This caused ImportService to use the previous importer instead of attempting to use the URL.

To fix this problem, we set import_type to git whenever the import by URL parameters are used. This is mapped to a nil importer in Gitlab::ImportSources.

Closes #31589 (closed)