TS@next: with a tsconfig.json like

        "outDir": "built/",
        // rootDirs for resolution of relative imports
        "rootDirs": [
          "src",    // for authored sources
          "codegen" // for generated code
        ],

and src/a.ts and codegen/a_gen.ts I end up with
built/src/a.js and built/codegen/a_gen.js.

I want an output structure that collapses the trees
built/a.js and built/a_gen.js
It seems like --rootDir might work, but

1. try rootDir: src -> error TS6059 because all sources are expected to be under rootDir
2. try rootDir: . -> no change in behavior

I would like some combination of rootDir and rootDirs that flattens the output tree.
cc @robwormald @tbosch