Fix tree shaking

[benoitgrelard]

Fixes #1428

[benoitgrelard]

Ok, this isn't as simple to change as I thought…

Mostly because of all the tsconfig mapping, but now also turns out the jest mapping too for tests.
They're all really geared better towards solutions with index files.

So we have a few options:

• try and update the jest mapping (moduleNameMapper) to make it work, so explicitly listing each package one by one like we have to do in the tsconfig (I've tried that quickly and still had issues)
• name all of the source files index.ts(x) so that the index is the source file directly. The thing I don't love about this is the experience in the editor when everything is called index. That said, I tried that quickly too and it looks like vscode now shows the "folder" that isn't in common next to the name, so combined with the fuzzy search, maybe this isn't as much as a big deal as I think?

Any other ideas? Obviously, the best thing would be that the issue gets fixed quickly 😀