scala_maven_import_external and scala_version

[ittaiz]

Thanks to @jhnj's work the scala version story is starting to unfold.
This caused me to think that maybe we want to have scala_maven_import_external allow to specify the scala_version in a different field with semantic meaning.
What I'd really like is to have the macro take the scala_version by itself from someplace. This can't be a label since the macro is used for repository rules but maybe we can do a trick with a file that contains the version somehow.
cc @natansil @anchlovi @or-shachar

[natansil]

maybe it can get it from a toolchain somehow

[jhnj]

I tried something similar (having a repository macro get scala_version from somewhere):

Use a repository_rule that creates a scala_version.bzl file containing def scala_version(): return "some_version".

However, I didn't get that approach to work as the scala_version.bzl file needs to be loaded and loading happens before it is created by the repository rule.