Add library field (excluded_modules)

[nojb]

This PR (which arose from a local patch that we have at LexiFi) adds a new field to the (library) stanza: (excluded_modules). This field (which is similar to (private_modules)) specifies a list of modules that:

• are compiled when the library is compiled,
• are not "seen" by non-excluded modules,
• are able to "see" all modules (excluded and non-excluded),
• are not linked into the final library, nor installed, etc

In our setting inside LexiFi, we define a library which contains the main part of our application code where the exact list of modules can vary from client to client, so we generate the list of modules dynamically using (:include). However, we want to make sure that the rest of the modules (which are not "active" for the current client) continue to compile. This feature is useful whenever you have "inactive" modules that you wish to make sure they still compile.

This feature can be simulated by introducing a second library, but doing so incurs in a substantial penalty due to the unnecessary recompilations caused by Dune's coarse-grained inter-library dependency approximation, as well as extra linking steps which can be quite slow.

Doc and tests are included.

[rgrinberg]

At first glance, this seems like a rather niche feature that's not going to be beneficial to many users. On the other hand, it does seem like there's something missing here in dune. Is it so difficult to keep this patch in your fork while we're still thinking of a way to generalize this feature?