Get fullName post flatten, to get flatten names

[dwijnand]

The reason this doesn't affect Scala 3 is because Scala 3 calls the zinc callback to register products during it's last, JVM bytecode emitting, phase. That affects pipelining, but I left problem for another day. Meanwhile, I found that the name is changed, in both compilers, in the "flatten" phase, so if I phase travel to that phase, getting the name (which is a recursive emission, through the ownership chain) will do the necessary "compatify" of long names.

Initially I tried compacting the name we already had, but the names didn't line up, because it was compacting the final name, rather than one of the owner names (as well as duplicating the implementation of compatify). That's also why I'm introducing checkProductsExist - to assert not only that the metadata matches the test expectations, but also the metadata matches the filesystem.

[lrytz]

Once this is merged it needs to be ported to https://github.com/scala/scala/blob/2.13.x/src/sbt-bridge/scala/tools/xsbt/CallbackGlobal.scala

[eed3si9n]

I am planning to release 1.9.x soon, so I'll merge this as-is.

[SethTisue]

tracking apparent regression at scala/bug#12868

[SethTisue]

revert PR #1256