Source compatibility with newer cmdliner versions

[WardBrian]

Related to issues #6424, #6849, #6853

This PR makes the necessary code changes to allow compilation under cmdliner 2.0 while keeping the existing vendored version of 1.3.0.

These changes are mostly find-and-replace type things, but they would allow (at some future point):

• Updating the vendor to 2.1.0+
• Progressive enhancement from that point (e.g., replacing of the Arg.conv' calls this PR ads with 2.0's Arg.Conv.make, which would also allow us to specify completion handlers)

---

To verify, change the vendor_cmdliner.sh script to

#!/bin/sh

set -euo pipefail

cd src/core/cmdliner
rm -rf *.ml *.mli dune

git clone https://github.com/dbuenzli/cmdliner tmp-vendor
git -C tmp-vendor switch --detach v2.1.0

mv tmp-vendor/src/*.{ml,mli} .
rm -rf tmp-vendor

mv cmdliner.ml opamCmdliner.ml
mv cmdliner.mli opamCmdliner.mli

cat > dune << EOF
(library
 (name opamCmdliner)
 (public_name opam-core.cmdliner)
 (flags :standard -w -27-32-35-50))
EOF

run it, and re-build.

[kit-ty-kate]

Thanks a lot! After some minor reformatting to avoid unrelated changes and simplify the diff it looks great.

Note for the remaining reviewer (@rjbou): the github "Hide whitespace" setting makes the diff straightforward

[WardBrian]

Thanks!

I wasn't sure if this kind of thing needed a CHANGES entry -- should I just ignore the CI run for that, or do you have a recommendation for what section I should put it under ("Internal"?)

[kit-ty-kate]

I wasn't sure if this kind of thing needed a CHANGES entry -- should I just ignore the CI run for that, or do you have a recommendation for what section I should put it under ("Internal"?)

you can ignore the CI, that's no problem, we'll do it before the last rebase/final merge. But if you want to do it before that, this one should probably go under Internal in the master_changes.md, indeed.