cadabra2 @2.5.4: inaccurate error claiming nlohmann-json is installed

[ryandesign (Ryan Carsten Schmidt)]

cadabra2 fails to build:

****
**** cadabra2 uses its own bundled version of nlohmann/json.hpp
**** The build will fail if the nlohmann-json port is active.
**** Please deactivate it:
****     sudo port deactivate nlohmann-json
**** You can reactivate it after installing cadabra2:
****     sudo port activate nlohmann-json
****

Error: Failed to configure cadabra2: nlohmann-json installed.

nlohmann-json is not installed. This occurs on the buildbot workers and nlohmann-json is not in cadabra2's recursive dependencies and non-dependencies are always deactivated before builds on the buildbot.

Note that the portfile code only prints this error if ${prefix}/include/nlohmann/json.hpp does not exist, therefore the port can currently only be installed if nlohmann-json is installed and trace mode is not used.

If cadabra2 cannot be built when the nlohmann-json port is installed, the conflicts_build portgroup should be used rather than reinventing the wheel in the portfile.

[Marius Schamschula <mschamschula@…>]

In e57f9a1dbdbba2405587dd92e49db26ce7fae2a8/macports-ports (master):

cadabra2: use conflicts_build PG

Closes: #70528