Re-allow .install files containing '..'

[kit-ty-kate]

Partially reverts ae877c0 (from #6879)

This check is no longer necessary since d7f7d8e (#7005) and broke opam repositories providing cross-compiled packages as such format is used by dune (see ocaml/dune#14393)

See https://ocaml.zulipchat.com/#narrow/channel/527832-opam/topic/Cross.20compilation.20with.20opam.202.2E5.2E1/with/592359165

Backported to 2.5 in #7009

[rjbou]

Also, I think it is better to avoid having links, especially non permanent ones on commit messages. If the message is deleted, or the zulip is no longer online, it is not possible to find out the origin that you wanted to highlight.

[rjbou]

Even if it wasn't enforced in the code, from the manual, .. are forbidden in .install target.
My opinion: I'm in favor or removing it (keep as is), especially as there is now a field that permit to no longer need them (#6938). For backward compatibility, we can reintroduce it, but discourage its use with a warning (that errors on strict mode). We can even think about removing the support in a few releases (we need then to give the information from now on release blog posts).

[kit-ty-kate]

Per #7009 (comment), that backward compatibility is required for 2.6 at least (then we can bring back the stricter rule in 2.7)

[rjbou]

I've update the code :

• rebased
• rebase on top of 7015
• added a warning (error on strict mode) for .. in target
• added a test with strict mode to highlight the error

[kit-ty-kate]

Ignoring the missing macOS runs. Seems like GitHub have nuked their macOS machines out of existence