Performance regression in the flat classpath

[smarter]

scala/bug#10289 should affect us too now that we've update our classpath implementation (#2191).

[smarter]

There is now a fix in scalac for the aforementioned bug: scala/scala#5956 The culprit turned out to be findClass which is used by the sbt scalac bridge. Since the sbt dotty bridge does not use findClass, it is likely that we are not affected by this regression after all, but we should still port the classpath implementation changes.

[smarter]

Fixed in #3116