Fix(pkg output): Fix the pkg output path

[kurikomoe]

Fix the problem when outDir is a relative path, wasm-pack will use
Cargo.toml as root path, but this._makeEmpty will use package.json

for example

webpack.config.js:

    new WasmPackPlugin({
      crateDirectory: path.resolve(__dirname, "./src/native"),
      outDir: "./source/pkg",
    }),

without the patch, extra index.js will be generated.

./source/pkg/index.js (empty file)
./src/native/source/pkg/xxxx (actual files)

[serprex]

drager/wasm-pack#712

It's actually a bug that wasm-pack isn't putting pkg where outDir expects it to go