javac plugin: keep parameter names

By adding `-parameters` when we build java code using the java compiler plugin we retain parameter names and therefore we get:

```
  func sayHello(_ x: Int32, _ y: Int32) -> Int32
```

rather than

```
  func sayHello(_ arg0: Int32, _ arg1: Int32) -> Int32
```

which is a slightly nicer developer experience.

When using with already built jars we can't influence that, but whenever using with java sources we actually build we should recommend keeping that flag probably.

Author: ktoso

Plugins/JavaCompilerPlugin/JavaCompilerPlugin.swift

@@ -58,7 +58,8 @@ struct JavaCompilerBuildToolPlugin: BuildToolPlugin {
           .appending(path: "bin")
           .appending(path: "javac"),
         arguments: javaFiles.map { $0.path(percentEncoded: false) } + [
-          "-d", javaClassFileURL.path()
+          "-d", javaClassFileURL.path(),
+          "-parameters", // keep parameter names, which allows us to emit them in generated Swift decls
         ],
         inputFiles: javaFiles,
         outputFiles: classFiles