Proposal to improve mill init support for importing projects using other build tools

[ajaychandran]

Some suggestions to improve the implementations added in #3756, #4363, #4586.

General

1. Update filter in BuildGenUtil.buildFiles to discard files based on name and not just extension. This is to avoid accidental re-formatting of, say, *.sc scripts in a project.
2. Use git to set up the workspace in BuildGenTestSuite.integrationTest. This better emulates real world usage and avoids the copying in the current implementation.

SBT

1. Add support for importing cross Scala version and/or cross Scala platform projects: I did get this working (partly) but I had refactored the existing IRs and added a few new ones. Will check if the POC can be ported with minimal disruption.
2. Add sbtOptions: mainargs.Leftover[String] to SbtBuildGenMain.Config. Among other benefits, MillInitSbtUtils.bumpSbt can be replaced with --sbt-version <version>.
3. The SBT apply-script-sideloading approach should eliminate the platform specific load.

Perhaps these can be handled in 2 separate PRs?