Add an option to enable libraries on specific contexts based on the modes field

[jchavarri]

In a project with both native and Melange libraries, one might want to keep the former in default context, and move all Melange libraries to a different context melange, so that library name collisions can be avoided (see related issue #10222).

To do this, Dune currently requires going through all libraries that include (modes melange), and add (enabled_if (= %{context_name} "melange")).

Besides that, if one wants to properly separate namespaces, all existing OCaml libraries should be annotated with the equivalent (enabled_if (= %{context_name} "default")).

This is quite noisy for projects with tens or hundreds of libraries. Besides, the enabled_if conditions are mostly redundant with the information already available in the library modes field.

I wonder if there could be some setting in such scenarios where a single addition to the dune-project or dune-workspace file could include the (= %{context_name} "xxx") condition to each library, depending on its mode, e.g.:

• If the library has (modes melange): adds (enabled_if (= %{context_name} "melange"))
• If the library doesn't have (modes melange): adds (enabled_if (= %{context_name} "default"))
• If the library has (modes :standard melange): doesn't add any condition (library is available in both contexts)

This setting could be activated through a new stanza (melange_use_mode_as_context true).

cc @anmonteiro @davesnx @andreypopp