scalafix-core_3 (GSoC 2022)

[rvacaru]

This PR is the first step towards adapting ExplicitResultTypes for scala3 #1583 and part of the plan for scala3 support #1680. The idea is to cross compile all scalafix modules to scala3 and then introduce a new ExplicitResultTypes implementation for scala3.

In this first step we're cross compiling scalafix-core to scala3

[bjaglin]

Another note: since core3 is an orphan modules not used by any other, I don't think its compilation is covered by CI, so you should add an explicit unit3/compile in

scalafix/project/ScalafixBuild.scala

Lines 131 to 134 in 59ed8c0

	commands += Command.command("ci-3") { s =>
	"unit2_12Target3/test" ::
	s
	},

This will apply for all xxx3 modules until we move unit2_12Target3 to unit3.

[bjaglin]

Last round of comments, and we'll be good to go (provided core3/compile is green!)