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 https://gitlab.com/gitlab-org/gitlab/-/issues/31589