fix: provide original code in source maps when using RAM bundles

[joeblynch]

Summary

When building RAM bundles, there are a couple issues which occur with the source map:

1. The source map contains the transformed source code output of Babel, instead of the original source code.
2. The original file paths in the source map are prefixed with node_modules/@haul-bundler/core/build/webpack/loaders/babelWorkerLoader/index.js??ref--5-0!.

Setting the webpack config to use devtool: 'source-map', when building RAM bundles, appears to resolve these issues.

Test plan

1. Build a RAM bundle of an RN app with Haul and produce a source map of the bundle. For reference I've included
   source maps.zip, which includes source maps from the default react-native init Android app, both with and without this fix.
2. Examine the content of the source map. To help with this, I threw together dump-source-map, which can be used to reconstruct the original source code, as contained in a source map file.

For example, without the fix, the source map will contain:
/Users/jlynch/tmp/TestRNApp60/node_modules/@haul-bundler/core/build/webpack/loaders/babelWorkerLoader/index.js??ref--5-0!/Users/jlynch/tmp/TestRNApp60/App.js

...
const App = () => {
  return /*#__PURE__*/React.createElement(Fragment, {
    __source: {
      fileName: _jsxFileName,
      lineNumber: 29,
      columnNumber: 5
    }
  }, /*#__PURE__*/React.createElement(StatusBar, {
    barStyle: "dark-content",
    __source: {
      fileName: _jsxFileName,
      lineNumber: 30,
      columnNumber: 7
    }
  }), /*#__PURE__*/React.createElement(SafeAreaView, {
...

With the fix, the source map will contain:
/Users/jlynch/tmp/TestRNApp60/App.js

...
const App = () => {
  return (
    <Fragment>
      <StatusBar barStyle="dark-content" />
...