The behaviour of opam show of multiple packages is inconsistent

[kit-ty-kate]

This is morally a reopening of #2980 issue 1.

Currently opam show ocaml ocaml-base-compiler will show both packages just fine.
But opam show ocaml.5.4.0 ocaml.5.3.0 will not work and even opam show "ocaml>=5.3.0" "ocaml<=5.4.0" that one would expect to work if formula were merged using and does not work as expected either as it will always display the same package twice (or how many times the package is present as argument):

$ opam show ocaml.5.4.0 ocaml.5.3.0
[ERROR] No package matching ocaml.5.4.0 or ocaml.5.3.0 found
$ opam show "ocaml>=5.3.0" "ocaml<=5.4.0"

<><> ocaml: information on all versions <><><><><><><><><><><><><><><><><><><><>
name                   ocaml
all-installed-versions [...]
all-versions           3.07  3.07+1  3.07+2  3.08.0  3.08.1  3.08.2  3.08.3  3.08.4  3.09.0  3.09.1  3.09.2  3.09.3  3.10.0  3.10.1  3.10.2  3.11.0  3.11.1  3.11.2  3.12.0  3.12.1  4.00.0  4.00.1  4.01.0  4.02.0
                       4.02.1  4.02.2  4.02.3  4.02.4  4.03.0  4.03.1  4.04.0  4.04.1  4.04.2  4.04.3  4.05.0  4.05.1  4.06.0  4.06.1  4.06.2  4.07.0  4.07.1  4.07.2  4.08.0  4.08.1  4.08.2  4.09.0  4.09.1  4.09.2
                       4.10.0  4.10.1  4.10.2  4.10.3  4.11.0  4.11.1  4.11.2  4.11.3  4.12.0  4.12.1  4.12.2  4.13.0  4.13.1  4.13.2  4.14.0  4.14.1  4.14.2  4.14.3  5.0.0  5.0.1  5.1.0  5.1.1  5.1.2  5.2.0
                       5.2.1  5.2.2  5.3.0  5.4.0

<><> Version-specific details <><><><><><><><><><><><><><><><><><><><><><><><><>
version     5.3.0
[...]

<><> ocaml: information on all versions <><><><><><><><><><><><><><><><><><><><>
name                   ocaml
all-installed-versions [...]
all-versions           3.07  3.07+1  3.07+2  3.08.0  3.08.1  3.08.2  3.08.3  3.08.4  3.09.0  3.09.1  3.09.2  3.09.3  3.10.0  3.10.1  3.10.2  3.11.0  3.11.1  3.11.2  3.12.0  3.12.1  4.00.0  4.00.1  4.01.0  4.02.0
                       4.02.1  4.02.2  4.02.3  4.02.4  4.03.0  4.03.1  4.04.0  4.04.1  4.04.2  4.04.3  4.05.0  4.05.1  4.06.0  4.06.1  4.06.2  4.07.0  4.07.1  4.07.2  4.08.0  4.08.1  4.08.2  4.09.0  4.09.1  4.09.2
                       4.10.0  4.10.1  4.10.2  4.10.3  4.11.0  4.11.1  4.11.2  4.11.3  4.12.0  4.12.1  4.12.2  4.13.0  4.13.1  4.13.2  4.14.0  4.14.1  4.14.2  4.14.3  5.0.0  5.0.1  5.1.0  5.1.1  5.1.2  5.2.0
                       5.2.1  5.2.2  5.3.0  5.4.0

<><> Version-specific details <><><><><><><><><><><><><><><><><><><><><><><><><>
version     5.3.0
[...]

I think we can change this behaviour and instead show each atoms independently in all cases.
In fact i'm unsure allowing the full capacity of an atom (pkg, pkg.version, pkg<op>version, <local file>) is appropriate for this command. I think only allowing pkg, pkg.version and <local file> seems more appropriate.