Zap modalities to identity for mli-less files

[riaqn]

For .ml files without a corresponding .mli, previously we thought the generated .cmi will only be on the LHS of inclusion check, and therefore it's always better to infer the strongest modalities. This is obviously wrong, because another module could invoke module type of on the .cmi and use it on RHS of inclusion check. This has broken legacy code in practice.

This PR therefore zaps modalitites to identity, which is more legacy compatible.

Side note: In the future we should add two flags to override the default behavior:

• one flag on mli-less files, to indicate the direction of zapping when exporting cmx. It should error if .mli exists:

[@@@zap_modalities_to_floor]
(rest of the ml file)

• One flag on module type of, to indicate the direction of zapping when calculating the module type. It should error if the module already has a user-annotated module type:

module M = struct
  ...
end

module type S = module type of M [@@zap_modalities_to_floor]

[riaqn]

This change is unrelated to the PR. The test was out-of-sync, because val_modalities_floor.ml was skipped by make test. I have made it not skipped, by re-ordering things in that file so it starts with (* TEST.