Improve the linter

[kit-ty-kate]

E.g.

• Check that depopts has no filters
• Check that on new packages there is no upper-bounds
• Check that > and <= are not used in constraints
• Check that = is not used in constraints (except for {= version})

[shonfeder]

More context from @mseri:

The only ones that I would like as warning but should not fail are

• Check that on new packages there is no upper-bounds
• Check that = is not used in constraints (except for {= version})

Because there are packages that use both of those for reasonable reasons
But

• Check that > and <= are not used in constraints
• Check that depopts has no filters

are bounds that we tripped over multiple times and would really be useful. The second is actually wrong all the times it happens

[shonfeder]

re: he "filters on depopts", what we need is

a lint that checks that no bound is on the depopts (They should be marked at conflicts)
(see ocaml/opam-repository#27033 for a case)

That is, instead of inclusive bounds on depopts, one should mark which packages are out of bonds as conflicts.