[pdal] Prepare for nlohmann-json 3.11.x

[falbrechtskirchinger]

In nlohmann-json 3.11.0, json_fwd.hpp changed.
Replace PDAL's internal copy of the file as part of the de-vendoring process.

This PR is a requirement for #26124 to move forward.

• Which triplets are supported/not supported? Have you updated the CI baseline?

  No change, No

• Does your PR follow the maintainer guide?

  Yes

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  Yes

[Osyotr]

There's no mention of nlohmann-json in pdal's vcpkg.json

[falbrechtskirchinger]

I'm confused about that as well, but it clearly references nlohmann/json.hpp. 🤷‍♂️

And nlohmann-json (3.11.x) fails CI as a consequence.

[falbrechtskirchinger]

I've no experience with vcpkg and don't use it myself, but I do get the impression there's more wrong with this port.

• Using "${CURRENT_INSTALLED_DIR}/include/nlohmann/json.hpp" without nlohmann-json being listed as a dependency.
• Using "${CURRENT_INSTALLED_DIR}/include/nanoflann.hpp" without nanoflann being listed as a dependency.
• Replacing the bundled single-header json.hpp with the multi-header version.

I could go on but let's start here.

[Osyotr]

Using "${CURRENT_INSTALLED_DIR}/include/nlohmann/json.hpp" without nlohmann-json being listed as a dependency.
Using "${CURRENT_INSTALLED_DIR}/include/nanoflann.hpp" without nanoflann being listed as a dependency.

nlohman-json dependency is implicitly satisfied: (pdal -> libgeotiff -> nlohman-json). But it should be explicit.
nanoflann is listed as a dependency of pdal https://github.com/microsoft/vcpkg/blob/39dbc6c45ba9493f1c1b59496d619e0d08f20436/ports/pdal/vcpkg.json#L20

[falbrechtskirchinger]

nanoflann is listed as a dependency of pdal

Right. I must have been looking at the wrong file. Thanks for clarifying, I'll add nlohmann-json explicitly.

I'm not a fan of conflating the bundled single-header json.hpp with the installed multi-header json.hpp, but it seems to work.
I believe I understand now why it works as well and that makes my other concerns moot.

[BillyONeal]

Thanks!