Don't compile web.cordova unibuilds unless needed

Specifically, we only compile them if there's a cordova platform in the
current project, or if we are publishing the package.

(Ideally, we wouldn't require every published package to have
web.browser and web.cordova unibuilds --- we'd just publish a 'web'
unibuild unless there's actually a difference between the two. But we
are not there yet.)

This adds an extra flag to isopack-buildinfo.json, so that we know to
rebuild all the isopacks when we add the first cordova platform (or
remove the last cordova platform).

The implementation around publish is a little clunky; if you're in a
non-Cordova app and run meteor publish, it will rebuild all the packages
with web.cordova, and the next time you prepare the app it will rebuild
them again without it. It does work though.

Fixes #3274.