Simplify init step for empty projects

[samoht]

When you want to create a new project, you need to have OCaml installed in your switch:

❯ opam switch create . --empty
❯ ocaml-platform
ocaml-platform: [ERROR] No compiler installed in your current switch.
❯ opam install ocaml
[...]
Done.
❯ ocaml-platform
[...]
* Success.

That might be useful at one point to automatically run opam install ocaml if the switch is empty (and eventually, run opam switch create . --empty if there is no local opam switch present yet - it's clearly a bit tricky to get that one right, so need to design this feature first and decide if that's a good idea or not).