Fix cli regression on switch creation with packages

[rjbou]

Fix #4843

$ OPAMCLI=2.0 opam switch create . 4.12.0+options --packages=ocaml-option-flambda --show

<><> Installing new switch packages ><><><><><><><><><><><><><><><><>
Switch invariant: ["ocaml-variants" {= "4.12.0+options"} "ocaml-option-flambda"]
The following actions would be performed:
  ∗ install base-bigarray        base
  ∗ install base-threads         base
  ∗ install base-unix            base
  ∗ install ocaml-option-flambda 1
  ∗ install ocaml-variants       4.12.0+options
  ∗ install ocaml-config         2
  ∗ install ocaml                4.12.0
===== ∗ 7 =====

$ opam switch create . 4.12.0+options --packages=ocaml-option-flambda --show 
[ERROR] Individual package and option '--packages' can not be specified at the same time. Use '--packages' or '--formula instead', e.g.
        opam switch create flambda --packages=ocaml-variants.4.12.0+options,ocaml-option-flambda

[dra27]

The msvc64 failure on AppVeyor can be ignored for now

[dra27]

Suggestion for tweaking the example shown - open bug reports on other ones suggest users really do read them occasionally ☺