Codegen: separate Android/Cxx/iOS modules codegen outputs

Summary:
For now, separate the definition of `modules` generator per platform to avoid file output collision. Additionally:
* For Android, produce files under java/ (plus nested subdirs based on packageName) and jni/ (for C++ files) - JavaPoet version already does it
* Allow configuring packageName for Android - JavaPoet version has this
* Avoid tmp directory dance in the CLI script, given the proper modules separation

Changelog: [Internal]

Reviewed By: hramos

Differential Revision: D24410864

fbshipit-source-id: 9bd6bc1d65bec037bfca32ec478f3af50d72e927

packages/react-native-codegen/src/generators/components/GeneratePropsJavaInterface.js

@@ -209,6 +209,7 @@ module.exports = {
     libraryName: string,
     schema: SchemaType,
     moduleSpecName: string,
+    packageName?: string,
   ): FilesOutput {
     const files = new Map();
     Object.keys(schema.modules).forEach(moduleName => {