Add chisel plugin to mill build system.

[sequencer]

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• new feature/API

API Impact

None

Backend Code Generation Impact

None

Desired Merge Strategy

• Rebase: You will rebase the PR onto master and it will be merged with a merge commit.

Release Notes

Add chisel-plugin to mill build system.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (3.2.x, 3.3.x, 3.4.0, 3.5.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?

[sequencer]

@azidar I moved src-2.12 to src, since chisel3 is a little tricky to support which, is any reason to do so?

[jackkoenig]

src/main/scala-2.12 is SBT standard way of making sure something only compiles when using Scala 2.12, Mill probably has support for something similar.

[sequencer]

fix with CrossSbtModule, but huge change in the build.sc :p

[edwardcwang]

Seems fine to me.