Introduce -Xoutput-imports mode of JVM compiler

In this mode, instead of analyzing files and generating bytecode for
them, compiler just saves imports of each file in JSON map of form

'<path to file> -> [<import1>, <import2>, ...]'

It is needed for some external tools, notably for Google3 toolchain.

compiler/compiler.pro

@@ -216,4 +216,7 @@ messages/**)
 # remove when KT-18563 would be fixed
 -keep class org.jetbrains.kotlin.psi.psiUtil.PsiUtilsKt { *; }
 
+# for imports dumper in compiler
+-keep class kotlinx.serialization.** { *; }
+
 -keep class net.jpountz.lz4.* { *; }