Fix issues with _3 and _2.13 dependencies

[lolgab]

Fixes #1241
Having a sane classpath using the Scala 2.13 compiler to compile a Scala 3 project is hard.
This uses some tricks to compile using Scala 2.13 dependencies (since we need the Scala 2.13 macros) and then change the artifacts with Scala 3 artifacts so the Ammonite runtime can compile the macros using the Scala 3 compiler.

[mpollmeier]

Just tested: @main entrypoints don't quite work for scala3 yet with this branch.
With #1298 it works, so maybe you can compare the runtime dependency trees..?

@main def main(name: String) = {
  println(s"Hello, $name!")
}

mill -i "amm[3.2.0].assembly" && out/amm/3.2.0/assembly.dest/out.jar test.sc --name Michael

Script test.sc does not take arguments: "--name" "Michael"

N.b. removing the --name argument doesn't change anything - the @main isn't found either.

[lolgab]

I didn't fix the @main entrypoints here. The idea was to fix the classpath first and then after this is merged you can rebase #1298 and it works out of the box without having to touch the build.sc file :)
Tried to merge your branch with this locally (excluding the build.sc changes) and your example correctly prints "Hello, Michael!" :)

[mpollmeier]

Oh right, sorry I didn't get that. Yes I can confirm, merging this and then a slightly modified version of #1298 gives us @main entrypoint support for Scala 3 🎉

[lefou]

Interesting approach. I think we can incorporate this, but I find the new use_3 object quite confusing right now. I hope I find more time later to better understand the code. Would be nice, if we could avoid the many indirections and lookups and the inner deps method in favor of some more explicit defs each resulting in single concrete ivy dependency.

[lolgab]

I get the dependencies via string so it's impossible to add a dependency without also adding it to the depsNames list which is used to patch the compile-time versions.

[lefou]

I get the dependencies via string so it's impossible to add a dependency without also adding it to the depsNames list which is used to patch the compile-time versions.

@lolgab I tried hack down what I had in mind. Can you please have a look at #1302 and let me know, whether this improves readability of the dependency setup?

[lolgab]

Thank you @lefou. I merged your changes. As you said, few people touch this repo and we can add the dependencies manually to the list.
It's ready to merge from my side.

[lefou]

Looks good to me. The final goal should be still to support Scala 3 directly, but it's a nice trick to get it work now. Thank you!

[mpollmeier]

The only thing left now is to reenable mainargs for Scala 3:
#1303

Thanks for sorting out the build dependencies @lolgab and @lefou!