Add back scala-collection-compat for Scala 2.12

[MaximeKjaer]

As per the discussion in #7, where @vincenzobaz wrote:

If my understanding is correct, scala-collection-compat is needed when we compile in 2.12 to use features from the 2.13 standard library (not only collections) in a 2.12 codebase.

Naturally, this dependency is compiled for 2.13 and 3.0.0-RC1 and could be used without cross.
However scio-core depends on algebird-core with .cross(CrossVersion.for3Use2_13) which depends on scala-collection-compat_2.13 which results in conflicts.

I believe that the next step for this is to add a if (is scala 2.12) { depend on it }. But I prefer to do it when I encounter the missing lib rather than now for all modules.