Failure to properly handle directory separators on windows platforms

[nberth]

The following behavior can be observed on Windows :

$ opam install dir/*.opam
opam: PACKAGES… arguments: Invalid character '/' in package name "dir/package"
Usage: opam install [OPTION]… [PACKAGES]…

This is surprising since this platform supposedly has handled both / and \ as directory separators for a while.

Versions used:

• opam.2.5.1 / mingw64
• ocaml.4.14.2
• (please ask for more if needed, I am not versed on what Windows-specifics may influence this behavior)