Transition mode for library wrapping: emit deprecation warning

[hhugo]

Discussed with @diml offline.

Switching to library from (wrapped false) to (wrapped true) usually mean that you break most package depending on you. Dune should support a transition mode that will emit warning if modules are accessed directly.

In that mode, for every module in the lib:

• dune would generate {MOD}.ml-gen
• compile {MOD}.ml-gen to {MOD}.cm{i,o,x}
• include these new implementations in the archive.

{MOD}.ml-gen files would look like:

include Lib__{MOD} [@@deprecated "Use Lib.{MOD} instead."]

[ghost]

The transition mode could also take a date:

(wrapped (transition_until 2017-12-00))

this date could be included in the deprecation message and at this date we could notify the author of the library that they need to switch to wrapped false.