SwiftPM command plugin to ingest a Jar file into a SwiftPM target to wrap those Java classes

The SwiftPM build tool plugin Java2SwiftPlugin wraps up the specified set of Java classes so they can be used from Swift. It requires a configuration file containing the mapping from specific Java canonical class names to their Swift type names. However, we'd like to make it easier to get started by importing a Jar file into a Swift target, scanning the Jar file for any public classes and generating the Java2SwiftPlugin configuration file.

A SwiftPM command plugin would be a reasonable approach here. Essentially, it would take a SwiftPM target name and Jar file and update the package to wrap the public classes from that Jar file. It could be written as:

swift package import-java-jar MySwiftTarget MyJavaLib.jar

This command would produce two outputs:

1. A file Sources/MySwiftTarget/Java2Swift.config that includes the Jar file in the class path and lists the mappings for all of the public classes within that Jar file.
2. An updated Package.swift that contains a package dependency on swift-java and applies the Java2SwiftPlugin to the MySwiftTarget.

The goal should be that one can "ingest" a Java library with one command, then swift build to get access to that library from Swift.

Start with the SwiftPM command plugin documentation and the existing Java2SwiftPlugin. Define the import-java-jar command around the specified target and Jar file, producing Sources/MySwiftTarget/Java2Swift.config and updating Package.swift as described. Done means the generated configuration covers public Jar classes and swift build can use the wrapped library.