support debugging for dotc/dotr

[liufengyun]

Support debugging option for dotc/dotr:

$ dotc -debug hello.scala
Listening for transport dt_socket at address: 5005

$ dotr -d Test
Listening for transport dt_socket at address: 5005

With this PR, we may consider remove the bin/dotc scripts:

• New scripts don't depend on source code
• New scripts support all features of old scripts
• New scripts is CI tested on Linux/Mac/Windows
• sbt dist/pack or sbt dist-bootstrapped/pack can be used for compiler development

The CI on Linux/Mac/Windows for this PR is here: liufengyun/packtest#3

Note: -d is already used in the compiler for output directory, thus dotc has to use -debug.

[DarkDimius]

New scripts don't depend on source code

Does this mean that they may run a "dirty" compiler that doesn't match the compiled thing?
If it's the case, I'll still keep the old scripts. I use them during local development

[liufengyun]

As I said, you can run sbt dist/pack or sbt dist-bootstrapped/pack to have two different versions of dotc/dotr at the same time for development.

• sbt dist/pack: generate bin under dist/target/pack/bin
• sbt dist-bootstrapped/pack: generate bin under dist-bootstrapped/target/pack/bin

My experience tells that it's easier and more flexible to work with the news scripts than the old ones for compiler development.

https://github.com/xerial/sbt-pack#features

Anyway, just a proposal to try the new ones. We don't remove old ones in this PR.