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/build.gradle.kts

@@ -20,6 +20,7 @@ dependencies {
     compile(project(":js:js.translator"))
     compile(project(":js:js.serializer"))
     compile(project(":js:js.dce"))
+    compile(commonDep("org.jetbrains.kotlinx", "kotlinx-serialization-runtime")) { isTransitive = false }
     compile(commonDep("org.fusesource.jansi", "jansi"))
     compile(commonDep("org.jline", "jline"))
     compile(files("${System.getProperty("java.home")}/../lib/tools.jar"))