opamfile: parse error on escapable paths

[rjbou]

Escapable path, containing .. now raise a parse error.

Removes lint 71 introduced in #5561

/cc @hannesm @reynir

• queued on Reftests: add tests to check url handling behaviours #5560
• queued on lint: add errors about duplicated extra-files and checksums & when extra-file paths contains '..' #5561

[AltGr]

A parse error on files is actually a pretty good idea, and a likely security improvement.

But enforcing the invariant at the OpamFilename level seems pretty strict to me, I am afraid that it could break a few things. Couldn't that affect, for example, commands like opam pin ../foo/foo.opam ? Or is the path always resolved and made absolute soon enough ?

[hannesm]

I looked at the last commit, and am convinced that it avoids .. being in a path. From a security point of view, I do appreciate this PR. But of course @AltGr question is valid - maybe the result is too strict?