Support mixing scala 2.13 and scala 3 macros

[gregghz]

Support mixing scala 2.13 and scala 3 macros as described here.

This would allow scala 2.13 and 3.x modules to depend on sourcecode_3 instead of conditionally choosing one or the other. The main upside is this makes incrementally upgrading to scala 3 possible.

To illustrate, assume I have the following 3 modules that all depend on sourcecode directly:

moduleA: a scala 2.13 project that depends on moduleB and moduleC
moduleB: a scala 2.13 library
moduleC: a scala 3 library

moduleB and moduleC must both depend on the same artifact (sourcecode_2.13 or sourcecode_3) or risk having two versions of sourcecode on the classpath at runtime (sourcecode_2.13 and sourcecode_3) when running moduleA.

Since scala 2.13 can't use scala 3 macros and scala 3 can't normally use scala 2.13 macros the only option would be to either leave all modules at scala 2.13 or ugprade all of them to scala 3 all at once. However, if the concept described in the link above is implemented, we would be able to depend on sorucecode_3 from all modules and still use either scala 2.13 or scala 3. This would make incrementally moving to scala 3 much easier, especially for a large code base with thousands of modules depending on sourcecode (directly and indirectly).