fix(packaging): use system nlohmann-json

[shiptux]

Summary

• make the nlohmann-json include root configurable while preserving the git submodule default for source builds
• build Debian and RPM packages against /usr/include and declare the matching distro build dependency
• fix RPM dependency selection: Fedora and RHEL/EPEL use json-devel; openEuler uses nlohmann-json-devel

Verification

• parsed setup.py with Python AST
• confirmed a FlagCX make dry-run emits -I/usr/include
• parsed the RPM spec for Fedora 43, RHEL 9, and openEuler macro sets
• verified package availability: Fedora 43/Rocky 9 provide json-devel 3.11.3; openEuler 24.03 provides nlohmann-json-devel 3.11.2

[MC952-arch]

LGTM