Include library JSON files in package distribution

Files like src/lib/libraries/sprites.json were being used by
packages like scratch-desktop in order to fetch all the library
assets for offline access. We stopped publishing src in #5263 to
stop bloating the dist with copies of src files. Explicitly
exporting the files that are needed by other repos is a better
long-term plan.

The files are now published to dist/libraries/*.json. Once this is
merged those repos that use the src/ path need to change it to e.g.
`require('scratch-gui/dist/libraries/backdrops.json')`