Friend modules should be provided using the -Xfriend-modules flag
in the same format as -libraries. No manual configuration required for
JPS, Gradle and Maven plugins.
Friend modules could be switched off using the -Xfriend-modules-disabled
flag. Doing that will
* prevent internal declarations from being exported,
* values provided by -Xfriend-modules ignored,
* raise a compilation error on attemps to use internal declarations from other modules
Fixes #KT-15135 and #KT-16568.
Vitaly