Deduplicate Fir2IrExtensions creation and move the
convertToIrAndActualizeForJvm function to 'cli', which allows to remove
the dependency of 'fir.entrypoint' on 'backend.jvm'.
Note that behavior slightly changes in IncrementalFirJvmCompilerRunner:
previously the value of linkViaSignatures was always false, now it is
taken from the compiler configuration, which seems more correct.