Issue with cross-building project that uses Avro classifier artifact

[clairemcginty]

I'm encountering an issue cross-compiling projects that import avro classifier artifacts and use default sbt-avro settings. Basically, the first compilation succeeds, but the second fails, because avroUnpackDependencies has cached the result for external-library-avro.jar... except that by default, these dependencies are unpacked into target dir target/scala-{VERSION}/src_managed.

Therefore, if we cross compile Seq(2.12, 2.13), the first compilation succeeds because the records are unpacked to target/scala-2.12/src_managed then compiled, but the second compilation fails because sbt-avro thinks that the unpacking is complete, but it looks for unpacked records in target/scala-2.13/src_managed, which is empty.

This is fixable by setting avroUnpackDependencies / target := target.value / "avro_external", but ideally it would work out of the box.

Maybe we can incorporate Scala binary version into the cache key if the value of avroUnpackDependencies / target contains -{scalaBinaryVersion.value}? Or just update the default value of avroUnpackDependencies outside of sourceManaged?

Repro here: https://github.com/clairemcginty/sbt-avro/tree/cross-compile-issue