Support not logop in <dep-specification>

[xguerin]

Desired Behavior

Support negating statements in the dependency specification grammar. and and or are already supported in the logop rule, and the negation is supported in the opam file format.

Example

(depends
 (ocaml (or (and :with-test (>= 4.08.0))
            (and (not :with-test) (>= 4.02.0)))

Workaround

(depends
 (ocaml (or (and :with-test (>= 4.08.0))
            (and (= :with-test false) (>= 4.02.0)))

[rgrinberg]

@avsm what do you think about this?

[avsm]

We should probably support the full opam grammar then; see also https://discuss.ocaml.org/t/compatibility-packages-conditional-dependencies-in-opam/6116/7.

I'd hoped to avoid the full complexity, but I think some of these formula are necessary these days.

[rgrinberg]

@avsm in that case, are you ok with my proposal at #3497?