Fail when trying to pin a package whose definition could not be found instead of forcing interactive edition

[kit-ty-kate]

Fixes #6322
This could happen with things like opam pin <typo> <url> or a typo in the pin-depends definition.

The interactive edition based on the dummy opam file template is now only allowed if the user is purposefully requesting it using --edit or opam pin edit.

[kit-ty-kate]

do we really want to keep this template around? In which situation is it useful? I feel like it doesn't really has any purpose and we might want to remove it entirely (probably in a separate PR)

[rjbou]

Discussion on dev meeting: Overall agreement on blocked CI issue. We can't set the editing only on non tty as it is used hackishly in several places (our test for ex.) with specific command in OPAMEDITOR (cp/sed/etc.).
This option was useful at the beginning of opam, to ease pinning packages that didn't have yet an opam file, but today almost all OCaml package have one. And if it is missing, it will mostly be added in the repository itself than just internally in users opam switch. But we need to keep the ability to do that, and opam pin add --edit can permit it.