support boolean literals in package dependencies

[v-gb]

Desired Behavior

In the depends fields of package stanzas of dune-project files, I'd like to be able to say true and false in places where variables like with-dev-setup are currently allowed. Currently, AFAICT, this is not possible (although you can write :false to get a variable named false, which I suppose kind of works, but is weird).

The context is my project vendors library L, and thus my dune-project needs to incorporate library L's dune-project dependencies. But L's dune-project contains references to with-dev-setup that I claim should always be off (because I'm not vendoring the tests, but I may use with-dev-setup for my own purpose). Obviously, I could manually simplify any boolean away, but the inclusion of L's dependency is done with a script.

Currently, I'm using (= 1 0) when I need false, but opam maintainers are not necessarily happy about that.

Example

I'd like to be able to write this sort of things in a dune-project:

(package
  (depends
   (alcotest
     (and
      false ;; <---
      (>= 1.3.0)))

[rgrinberg]

Do we accept things like (and pkg1 pkg2)? If we do, we'll need to escape false and true as :false or :true.