rewrite mill build scripts for chisel5.

[sequencer]

This PR cleaned up the old build script, split it to common.sc and builds.sc two files. common.sc is used for declare the dependencies and project structual for each modules, which can be reused by other projects for building from source. build.sc is used for build project in tree.

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• 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?
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API Impact

Backend Code Generation Impact

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Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
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[sequencer]

@jackkoenig request for review.

[jackkoenig]

As I mentioned to you on Slack, this needs to be rebased on #3174 which adds a new firtoolVersion: Option[String] to the BuildInfo. You can just make it None as it really only needs to have a value when publishing and we still use SBT for that (for now).

It's great to see this working again, please add it to CI so we know if it breaks in the future.

Also, I'm not necessarily against supporting multi-project builds binding Chisel from source via the mill rules, but I'm not keen on making it a supported API. I think anyone building from source will have to accept that the build rules can change at any time for any reason.

[ZenithalHourlyRate]

Can this be backported to branch 3.6.x? chisel 3.6.0 already adds firtoolVersion in BuildInfo.