Support incremental compilation with coverage instrumentation

[lolgab]

Currently, coverage instrumentation data gets deleted and recreated from scratch at every compiler run, regardless if it is an incremental compilation run.
This makes it impossible to reuse instrumentation data and incremental compilation and requires to run clean and recompile everything if you want to have reliable coverage information.
This happens in Scala 2 and in Scala 3.

There is already a ticket in the Scala2 scoverage plugin scoverage/scalac-scoverage-plugin#492, but since in Scala 3 coverage was moved inside the main compiler repository, I'm creating a companion issue here.

This makes it impossible to cache sbt target in CIs if coverage information is needed, making Scala continuous integration much slower than it should be (you need to recompile a whole codebase even if a single line was changed).