Turn depopts with constraints into conflicts

See https://github.com/ocaml/opam/pull/1325 : this will avoid lots of warning
and keep consistency when we merge

Rewrite done using:
```
opam-admin.top ../opam/shell/depopts_to_conflicts.ml
```
Rewrite '!=' version constraints for 1.1 compat

Rewrite done using:
```
opam-admin.top ../opam/shell/remove_notequals.ml
```

Still feels weird to replace 'a != b' by 'a < b AND a > b', but conflicts
are provided as conjunctions in opam files at the moment, and since this
is a backwards-compatibility fix...