Parcel does not split side effect free re-exports across bundles

[devongovett]

Reproduction

Many large libraries have many exports in an index file which are re-exports of other modules, for example a library of React components. If an application utilizes code splitting, and imports different components from different bundles, Parcel currently includes all used components in the entry bundle rather than only the components necessary. This is because the index file is marked as used, and with it comes all of its dependencies. We could be smarter about this and only include dependencies that have used symbols in the current bundle.