Fix #5812: Compactify names when flattening

[griggt]

This is intended to replicate the behavior of Scala 2.13, where classfile names are limited to 240 characters.

Fixes #5812

[test_windows_full]

[griggt]

I suppose we need to consider scala/scala#7483.

Also, should we test that the compactified names match those generated by Scala 2.13? (a cursory inspection of the classfiles created by t8199.scala seems to indicate that this is indeed the case)

[griggt]

The Scala 2 compat test succeeds on lampsrv26 but fails on lampsrv9 with java.nio.file.FileSystemException: ... File name too long.

Perhaps they are using different Docker storage drivers. The file name is currently 244 characters long, I'll shorten it slightly. I think the max for aufs is 242.

https://github.com/lampepfl/dotty/actions/runs/535610971

[smarter]

Thanks!