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/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JVMCompilerArguments.kt

@@ -225,6 +225,13 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
     )
     var noExceptionOnExplicitEqualsForBoxedNull by FreezableVar(false)
 
+    @Argument(
+        value = "-Xoutput-imports",
+        valueDescription = "<path>",
+        description = "Output imports from all compiled files to the specified file in JSON format"
+    )
+    var outputImports: String? by FreezableVar(null)
+
     @Argument(value = "-Xenable-jvm-default", description = "Allow to use '@JvmDefault' for JVM default method support")
     var enableJvmDefault: Boolean by FreezableVar(false)