Support canonical on module types, types, etc.

[lpw25]

Currently @canonical only works on modules. It is used to undo the namespacing games that dune does, but it is also useful more generally for cases where a module is exposed in a library at a different name. For example, something like this is a common pattern:

You have a foo0.ml:

module M = struct ... end
...

You have a foo.mli:

module M = Foo0.M
...

The library exposes Foo as e.g. Bar.Foo but does not expose Foo0. By default references to Foo0.M become unresolved links, but they can easily be fixed, by changing foo0.ml to:

(** @canonical Bar.Foo.M *)
module M = struct ... end
...

However, there is currently no way to fix the similar case where Foo0.M is a module type:

module type M = sig ... end
...

because @canonical does not work on module types.

[jonludlam]

Yes - I've been thinking about having it on types too to fix odoc's path types, which are currently declared in paths_types.ml but they're intended to be exposed in paths.ml

[jonludlam]

Fixed in #596