[build] Ensure isolation of Yojson wrt to Flèche

[ejgallego]

As of today, we want to ensure Flèche remains transport agnostic, however due to fleche_waterproof Yojson is in scope so that can be a bit tricky to ensure as we can pass some callback that expects LSP messages as we did before #513

There are two ways to fix this:

• use (implicit_transitive_deps false), but that doesn't work well at all I think
• make Fleche_waterproof register with Flèche, but this adds some overhead.