Make older Scala 2.12 versions work

[tgodzik]

Previously, we would only publish for the newest Scala 2.12 version, howeverthis might cause issues for versions older than 2.12.12. To help with that, I added a full cross version and made sure that a proper versions is picked up by the sbt plugin.

[olafurpg]

Thank you for doing this change! LGTM

just a thought, would it be possible to to add a library versioned artifact that is an alias for the latest version? That would remove the breaking change.

[tgodzik]

Thank you for doing this change! LGTM

just a thought, would it be possible to to add a library versioned artifact that is an alias for the latest version? That would remove the breaking change.

Actually, I think it is possible! I added:

  CrossVersion.binaryWith(
    prefix = "",
    suffix = if (scalaVersion.value == scala212Legacy) ".12" else ""
  )

which for binary cross version will add .12 as a suffix, so it will only have a full versions for 2.12.12. I made similar changes in the plugin and tested it out in Metals.

[tgodzik]

Tested it out with Metals so it seems to work, I will cut the new release and can fix anything that comes up later.