This change adds all information that is needed to generate the externs
file to build/info.json, so tasks/generate-externs.js no longer needs
to spawn JSDoc.
Because we don't know if a new or modified file includes changes to the class hierarchy, we regenerate info for all sources any time any one has changed. An alternative would be to generate info first for the new or modified file and then (potentially) regenerate info for more source files in the class hierarchy, but this makes the generate-info.js task far more complicated.
This task generates build related metadata for the library based on doc annotations. Since it is about more than writing out exportable symbols, it makes sense to have a more general name.
