Move gitlab-workhorse checkout to top level in GDK
For historical reasons, in GDK you find gitlab-workhorse in a deeply nested directory:
/path/to/gitlab-development-kit/gitlab-workhorse/src/gitlab.com/gitlab-org/gitlab-workhorse
This is bad for developer ergonomics and with go.mod
it's no longer necessary in any way.
We have fixed this problem for Gitaly not so long ago and I want to do the same for gitlab-workhorse.