Move the API project import files ee/lib -> lib

This moves some files back from ee/lib to lib, they're still identical in CE.

The file is still in it's original place in CE:

• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/project_import.rb
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/spec/requests/api/project_import_spec.rb