Plan for Dune 3.0

relying on the findlib configuration file I’m relying on findlib.conf to cross compile packages on esy, currently esy just sets the OCAMLPATH but we intended to move esy to findlib.conf, as it seems like a better way to setup the environment and it reduces the size of the environment.

I believe @anmonteiro is also using the same for cross compiling on nix.