Merge the sbt compiler bridge as a subproject of dotty

[smarter]

Note that if we want to run the bridge tests on every PR we should add the following to a Jenkins job:

sbt bridge/test
sbt bridge/scripted

The scripted tests take a fair amount of time (27 minutes here) so we may not want to do that right now, this will get better once we base ourselves on sbt 1.0.

[smarter]

Review by @felixmulder

[felixmulder]

LGTM (once those two comments have been answered)

[felixmulder]

Also, great job! I think we can all agree that you're the MVP of the week! 🎉

[DarkDimius]

I'd prefer to have commits in this PR squashed for several reasons:

• they do not show up on github;
• they make my local tooling(SourceTree) go into infinite cycle;
• the last commit in any case screws the blame history(at least in my IDE) so I don't see much value in maintaining it given that it breaks tooling(both on github and locally).

Otherwise LGTM.

[smarter]

the last commit in any case screws the blame history

git blame bridge/src/main/scala/xsbt/CompilerInterface.scala seems to display the correct thing.

Anyway, seems like you're trying to use a bunch of broken tools, feel free to change this PR to make them work with your tools, but maybe you should change tools ;)

[smarter]

If you do squash the commits, the commit message should at least include a link to https://github.com/smarter/dotty-bridge so that people can find the real history.