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/frontend.java/src/org/jetbrains/kotlin/config/JVMConfigurationKeys.java

@@ -103,7 +103,7 @@ public class JVMConfigurationKeys {
 
     public static final CompilerConfigurationKey<Boolean> PARAMETERS_METADATA =
             CompilerConfigurationKey.create("Parameters metadata for java 1.8 reflection");
-
+    
     public static final CompilerConfigurationKey<IncrementalCompilationComponents> INCREMENTAL_COMPILATION_COMPONENTS =
             CompilerConfigurationKey.create("incremental cache provider");
 
@@ -137,6 +137,9 @@ public class JVMConfigurationKeys {
     public static final CompilerConfigurationKey<List<String>> ADDITIONAL_JAVA_MODULES =
             CompilerConfigurationKey.create("additional Java modules");
 
+    public static final CompilerConfigurationKey<String> OUTPUT_IMPORTS =
+            CompilerConfigurationKey.create("output imports");
+
     public static final CompilerConfigurationKey<Boolean> ENABLE_JVM_DEFAULT =
             CompilerConfigurationKey.create("Allow to use '@JvmDefault'");
 }