Separate chunks for each module

Right now we only have `core.js` and `optional.js` as our output files.
We'd like to rename `core.js` to `amp-sw.js` and split `optional.js` to a single js file per module.

Start by tracing the build entry points that produce core.js and optional.js, then identify how modules are grouped into the optional bundle. The work is done when core.js is emitted as amp-sw.js and optional modules are emitted as separate JavaScript files, with the generated output checked for correctness.

Written by the indexing model from the issue text.