The dmd backend outta be a git sub module.

[Steven Schveighoffer]

On 7/1/20 2:06 PM, 12345swordy wrote:
> Having it as a sub module allows walter to full speed ahead without 
> waiting for Pull request approval. I don't see other backend developers 
> active besides walter here, on github.

The entire D CI ecosystem is based on dmd. If this weren't the case, it 
would be fine, but it's not.

In other words, we need to keep DMD building properly and working as 
designed, even if there is only one developer working on DMD backend.

-Steve