Revert "dev-ml/*: Remove := slot ops from || deps, #586304"

Revert "dev-ml/*: Remove := slot ops from || deps, #586304"

This reverts commit 2d4ac687fbf44588f764697c9dcb2b960a18d51b.

This commit is wrong in several aspects:
- Such changes should come with a revbump
- Removing := deps causes packages not to be rebuilt on upgrades, causing
  barely understandable build failures when upgrading, later on.
  Thanks to this commit, people that have installed those packages
  will likely experience those failures with the upcoming ocaml 4.03 unmask.
- This is a non maintainer commit where no discussion at all happened with the
  maintainers during the 3 days between bug report and mass commit.
  Such discussion could have prevented the above mentioned breakage.

Proper fix is to drop the backward compatibility part of the || () dep. This fix
was slowly moving forward whenever possible and noticed. Main blocker for this is
bug #463018 which requires taking extra care for not breaking the stable tree
nor needlessly preventing future stabilization of packages.