Add support for ppxlib 0.23.0

[kit-ty-kate]

ppxlib will come without OMP starting with ppxlib.0.23.0.
A few changes were needed to match the new module location with what metaquot expects.

cc @pitag-ha

[pitag-ha]

Update: Ignore this comment :) As pointed out below, it's wrong.

Thanks for the fix, @kit-ty-kate!

I was thinking if it would make sense to add a ppxlib < 0.23.0 constraint to metaquot instead of referencing the internal library Astlib__. I've just had a look and there are currently 6 opam packages that depend on metaquot. Two of them (ppx_pbt and traverse) practically don't seem to use it anymore, so we could (and should) also drop it from their opam file; the other four (clangml, override, refl and pattern) are based on ppxlib already, so if I'm not missing anything we could use ppxlib.metaquot there instead.

In general (also for unpublished packages), if the package is based on new ppxlib it should be able to use ppxlib.metaquot, if I'm not missing anything. Is that right? Do you know of any cases where it needs to use metaquot instead of ppxlib.metaquot, @thierry-martinez and @kit-ty-kate?

Also, @thierry-martinez, I've seen that 5 out of the 6 opam package I've mentioned above are yours: does it sound right to you what I've said about them?

[kit-ty-kate]

I was under the impression that metaquot and ppxlib.metaquot had different goals/features as it was created well after it were available (this project was created just last year), though I might be mistaken.

[pitag-ha]

Oh ok, thanks for pointing that out @kit-ty-kate, sorry for that!

[pitag-ha]

By any chance, is it possible to use the public path to Location, so Ppxlib_astlib.Location.t , instead of the private one, i.e. Astlib__.Location.t, here? And in case that's possible, same for Longident below.

[pitag-ha]

The ppxlib version @kit-ty-kate has mentioned above 0.23.0 has been released now. So for this metaquot extension to stay compatible with new ppxlib versions, it would be great to address this PR now.