Use version-aware naming for scala_compiler_source repository

[aszady]

Description

A small preparation for multiple SCALA_VERSIONS.

In theory breaking, as removes @scala_compiler_source repository.
But I guess it's too internal to anyone depend on it.

Motivation

Originally #1290.
Partitioned from #1552.

[aszady]

Hi @simuons, @liucijus,
can we do anything to help you reviewing these PRs regarding cross-build support?
Note that all open PRs are independent of each other and could be reviewed/merged in any order.

[simuons]

Hi @aszady, sorry last week I was on vacation and could not get over your PRs. I'm looking at them now and will answer today/tomorrow.

[simuons]

Hi, @aszady, could you confirm this is still relevant? I mean in another PR you mentioned that direction you are taking is to have single label with select. This doesn't apply here?

[aszady]

Yes; I think the general convention should be like:

• for repositories, that actually need to download something (artifacts, see: Allow version-aware naming for artifact repositories #1562; or here the compiler source) – use individual repository names with the suffix;
• for targets, that are usually built (e.g. majority of Don't rely on global SCALA_VERSION in BUILD files, use select instead #1564) – distinguish them by configurations + select.

Toolchains with config transitions seem to nicely join these two worlds together.

[simuons]

Thanks @aszady