See https://github.com/Kotlin/KEEP/issues/95#issuecomment-383889404
Drop Experimental.changesMayBreak, Experimental.Impact, the concept of
signature/body usage, same module exemption. Make the majority of tests
single-module because there is now no difference in the checker between
usages from the same module or from another module
This results in more diagnostics usually, but allows library authors to
avoid annotating everything in each experimental class with the marker
(only the class needs to be annotated now)
#KT-22759