Fix binary version computation of Scala nightly versions

[alexarchambault]

The Scala nighly versions look like 2.13.5-bin-f956b11 for example, and can be downloaded from here. AFAIU, their binary version should look like X.Y, just like stable versions, so that the binary version of 2.13.5-bin-f956b11 would be 2.13. This PR fixes that.

This would be useful for com-lihaoyi/Ammonite#1150, where using Scala 2 nightlies would allow to bump the Scala 3 version (from 3.0.0-M1 to 3.0.0-M3).

I'm not really sure if / where I should add tests for that.

[lefou]

Looks reasonable. I wonder if we should limit it to a major of 2. I can't point you right now to the best place for a test. I think we cover this only partially in intergration tests, but I think a simple unit test for def scalaBinaryVersion should do it.

[smarter]

Note that for Scala 3 the binary version should always be 3: https://github.com/sbt/librarymanagement/pull/374/files

[alexarchambault]

@smarter Even for the RCs?

[smarter]

For all RCs that will happen after 3.0.0 is out yes (so 3.0.1-RC1 is _3, but 3.0.0-RC1 is _3.0.0-RC1), because they won't break binary compatibility (but if they break tasty compatibility we'll bump the tasty version number).

[alexarchambault]

Right, I meant for the current RCs, yes.

[lefou]

Change looks good to me. We need to fix the tests, though.

[adpi2]

I am currently trying to fix the tests in #1259

[lefou]

Superseded by #1259 (contains the same commits).