Tag packages with missing depexts to the solver

[AltGr]

to allow playing with criteria and seeing if we can solve the
mariadb/mysql dilemma.

With this patch, you can add a -count[missing-depexts,solution]
criterion (putting it before the main version-lag criteria would be a
bad idea though).

[rjbou]

Tested in simple case

$ opam show foo -f depends # bmake is installed in the system, but no llvm
"ocaml"  ("conf-bmake" | "conf-llvm")

$ ./opam install foo.0.4 --show
The following actions would be performed:
  ∗ install conf-llvm 10.0.0 [required by foo]
  ∗ install foo       0.4
===== ∗ 2 =====

The following system packages will first need to be installed:
    llvm-10-dev

$ ./opam install foo.0.4 --show '--criteria=-removed,-count[version-lag,request],-count[missing-depexts,solution],-count[version-lag,changed],-changed'
The following actions would be performed:
  ∗ install conf-bmake 1.0 [required by foo]
  ∗ install foo        0.4
===== ∗ 2 =====