SwiftPM build plugin for producing Swift wrapper types for Java classes (Java2Swift direction)

[DougGregor]

The "Java2Swift" tool produces Swift wrapper types (using JavaKit) for Java classes. It's provided as a standalone executable that takes in an input module, some translation manifests, and either a Jar file or a list of class names, and produces Swift source files.

We should create a SwiftPM build plugin that runs the tool for us. For example, it should make it easy to declare a SwiftPM target and fill in some of the source files via the generator. For example, the JavaKitJar module is fully generated, but with a Makefile.

[DougGregor]

This was mostly implemented in PR #77 , but we need to make sure that we're picking up the manifest files generated from other targets we depend on. Right now, we only get the manifest files if they are in the "pre-baked" location generated/<modulename>.swift2java.

[DougGregor]

Done and documented