fix mill build

[sequencer]

Related issue: #1291 #1035

Type of change: bug report

Impact: no functional change

Development Phase: implementation

Release Notes
Bug fix for mill build system, original mill has been deprecated since the update of mill.
This implementation use native mill API as much as possible, which can keep better compatibility to which.

[claassistantio]

All committers have signed the CLA.

[sequencer]

As for accessing by other build.sc, for example there is a project structure in:

|proj
|-chisel3
|-firrtl

we can give a build.sc under proj,

import $file.chisel3.build
import $file.firrtl.build

object myfirrtl extends firrtl.build.firrtlCrossModule("2.12.10") {
  override def millSourcePath = super.millSourcePath / 'firrtl
}

object mychisel3 extends chisel3.build.chisel3CrossModule("2.12.10") {
  override def millSourcePath = super.millSourcePath / 'chisel3
  def firrtlModule: Option[PublishModule] = Some(myfirrtl)
}

object foo extends ScalaModule {
  def moduleDeps = Seq(mychisel3)
}

to make mychisel3 depend on myfirrtl. Then foo can depend on the mychisel3.

[edwardcwang]

Maybe add a line above this saying what mill version this has been tested with?

[edwardcwang]

Tested at https://github.com/edwardcwang/chisel-bleeding-edge-test and it appears to work exactly as expected.

[sequencer]

Any update for this PR?