Review and propagate part's `exists` conditions

[dbuenzli]

E.g. w.r.t. to a part's needs condition of existence. Difference between integrated and consulted. One example of where such a propagation occurs is in the Run.with_bin similar thinking should be done for the other parts. The exact propagation behaviour (which may be part dependent) should also be properly documented.

One simple example Bin and Lib parts of type ``OCamlshouldn't exist if bothConf.ocaml_byte` and `Conf.ocaml_native` are false.