Add goimports linter rule

What does this MR do?

Add goimport rule so that anything under gitlab.com/gitlab-org/gitlab-runner is in it's own import group. This is similar as if we are running goimports --local gitlab.com/gitlab-org/gitlab-runner

Why was this MR needed?

We often have this a comment in the merge request reviews that anything under gitlab.com/gitlab-org/gitlab-runner should be under it's own package. Instead of commenting on it every time make it part of the listing rules.

Every time something is not sorted as expected we see:

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?