This commit modifies the strategy to local src/lib rewrites, so that the
package.json files are never modified.

Intead, the package.json files always point at `bin/...`; which in turn
requires `lib/bin.js`.

During development, `lib/bin.js` can just require `src/bin.js` and rely on
require.extensions register. But on distribution `lib/**` can be pre-built.

By maintaining a stable `bin/...` entry-point, the following benefits are
achieved:

* The git workspace does not become dirty after a build
* It's not possible to get into a state where `node_modules/.bin/parcel` points
  at a non existant `lib/` file, which can only be fixed by removing
  node_modules and yarn installing again