packaging: consider using entry points to limit the number of possible imports

[tahini]

See https://nodejs.org/api/packages.html#package-entry-points

Currently, any file compiled in a package can be imported from other packages. But sometimes, the intent is to have an index.ts file in a folder which should be the one to include and the other files are just there to split the code. With entry points, we can avoid external packages including "intermediary" files.