The <package> Doesn't build with dune error is omitted from the output of opam monorepo lock in some cases

[gridbugs]

When a package depends on a non-dune package and a specific version of the compiler the error message displayed when running opam monorepo lock does not mention the fact that the failure was due to a dependency on a non-dune package. For example, locking the following package:

opam-version: "2.0"
depends: [
  "ansicolor"              # ansicolor does not depend on dune
  "ocaml" {= "4.14.1"}
]

The error message will be (the package name was "qux"):

opam_monorepo.exe: [ERROR] Can't find all required versions.
Selected: base-bigarray.base base-domains.base base-nnp.base
          base-threads.base base-unix.base ocaml-config.3
          ocaml-options-vanilla.1 ocaml-base-compiler&qux ocaml-base-compiler
          ocaml-base-compiler ocaml base-domains ocaml-base-compiler
- ocaml -> ocaml.5.0.0
    ocaml-base-compiler 5.0.0 requires = 5.0.0
- ocaml-base-compiler -> ocaml-base-compiler.5.0.0
    ocaml-base-compiler|ocaml-variants|ocaml-system ocaml-base-compiler requires >= 5.0.0~ & < 5.0.1~
- qux -> (problem)
    Rejected candidates:
      qux.zdev: Requires ocaml = 4.14.1

The only problem with this package is that the dependency ansicolor does not build with dune, but the error message could lead users in the wrong direction when debugging this issue.

The expected error message is (prior to 46b287f):

opam_monorepo.exe: [ERROR] Can't find all required versions.
Selected: base-bigarray.base base-domains.base base-nnp.base
          base-threads.base base-unix.base ocaml-config.3
          ocaml-options-vanilla.1 qux.zdev ocaml-base-compiler&qux
          ocaml-base-compiler ocaml-base-compiler ocaml base-domains
          ocaml-base-compiler
- ansicolor -> (problem)
    No usable implementations:
      ansicolor.0.5: Doesn't build with dune
- ocaml -> ocaml.5.0.0
    ocaml-base-compiler 5.0.0 requires = 5.0.0
- ocaml-base-compiler -> ocaml-base-compiler.5.0.0
    ocaml-base-compiler|ocaml-variants|ocaml-system ocaml-base-compiler requires >= 5.0.0~ & < 5.0.1~

Note the ansicolor.0.5: Doesn't build with dune, which is omitted in the original error message.

After commit 46b287f the error message is expected to be:

opam_monorepo.exe: [ERROR] Some dependencies cannot be built with dune!

opam-monorepo requires that all dependencies use dune as their build system.

These dependencies (possibly transitive) don't use dune as their build system:
- ansicolor

The dune-universe opam repository (git+https://github.com/dune-universe/opam-overlays.git) contains dune ports of some popular packages to help build more packages with dune however it appears to already be set up on this switch. Thus it is possible that no dune port exists for any of these packages.

For information on how to contribute a new dune port, see: https://github.com/dune-universe/opam-overlays

Removing the version constraint on the "ocaml" package causes the problem to go away.

Note that on the latest release of opam-monorepo (0.3.5) this error doesn't seem to happen, though opam monorepo lock succeeds with no error message and proceeds to generate a lockfile which doesn't download ansicolor at all, which I assume was a bug that has since been fixed.

My hypothesis for why this is happening is after the solver concludes that no solution is possible, it gets run again, passing ~closest_match:true (https://github.com/ocaml-opam/opam-0install-solver/blob/a17703ba83374975633d66bea2d40da51beabee5/lib/solver.ml#L57). The solver doesn't know that it's more useful to pick a partial solution whose diagnostics will include the fact that it failed due to a package having no versions that build with dune.