Make confict messages more precise

[kit-ty-kate]

With #6106 fixing #4373, many conflict messages now appear as they should be. However some things can still be improved. For example:

# Using opam-repository at cc50463b37f3ef495531c6623ef84118b9b78963
$ opam install pyml
[ERROR] Package conflict!
  * No agreement on the version of ocaml:
    - (invariant) → ocaml-base-compiler >= 5.2.0 → ocaml = 5.2.0
    - pyml → ocaml < 5.0
    You can temporarily relax the switch invariant with `--update-invariant'
  * No agreement on the version of ocaml-base-compiler:
    - (invariant) → ocaml-base-compiler >= 5.2.0
    - pyml → ocaml < 5.0 → ocaml-base-compiler < 4.14.3~
  * No agreement on the version of ocaml-base-compiler:
    - (invariant) → ocaml-base-compiler >= 5.2.0
    - pyml → stdcompat >= 18 → ocaml < 5.2 → ocaml-base-compiler < 5.1.2~
  * Incompatible packages:
    - (invariant) → ocaml-base-compiler >= 5.2.0
    - pyml → ocaml < 5.0 → dkml-base-compiler
  * Incompatible packages:
    - (invariant) → ocaml-base-compiler >= 5.2.0
    - pyml → ocaml < 5.0 → ocaml-variants
  * Missing dependency:
    - pyml → ocaml < 5.0 → ocaml-variants → ocaml-beta
    unmet availability conditions: 'enable-ocaml-beta-repository'

The first, second and three last can be confusing to users. The reason the solver returns that is that pyml.20220615 and older have the ocaml < 5.0 constraint but not the newer versions. I think it would help to output something like:

[ERROR] Package conflict!
  * No agreement on the version of ocaml:
    - (invariant) → ocaml-base-compiler >= 5.2.0 → ocaml = 5.2.0
    - pyml <= 20220615 → ocaml < 5.0
    You can temporarily relax the switch invariant with `--update-invariant'
  * No agreement on the version of ocaml-base-compiler:
    - (invariant) → ocaml-base-compiler >= 5.2.0
    - pyml <= 20220615 → ocaml < 5.0 → ocaml-base-compiler < 4.14.3~
  * No agreement on the version of ocaml-base-compiler:
    - (invariant) → ocaml-base-compiler >= 5.2.0
    - pyml → stdcompat >= 18 → ocaml < 5.2 → ocaml-base-compiler < 5.1.2~
  * Incompatible packages:
    - (invariant) → ocaml-base-compiler >= 5.2.0
    - pyml <= 20220615 → ocaml < 5.0 → dkml-base-compiler
  * Incompatible packages:
    - (invariant) → ocaml-base-compiler >= 5.2.0
    - pyml <= 20220615 → ocaml < 5.0 → ocaml-variants
  * Missing dependency:
    - pyml <= 20220615 → ocaml < 5.0 → ocaml-variants → ocaml-beta
    unmet availability conditions: 'enable-ocaml-beta-repository'