Handle local includes

[varac]

I include local files like this:

 include:                                                                                                                                                                              
   - .gitlab/ci_templates/kaniko.yml  

But gitlab-ci-validate doesn't handle them:

❯ gitlab-ci-validate .gitlab-ci.yml 
.gitlab-ci.yml is invalid:
  - Local file `.gitlab/ci_templates/kaniko.yml` does not have project!