setup.py: fix macOS dylib packaging and runtime linking

[Agrim-P777]

Summary

This PR improves cross-platform packaging and installation for pygenn, with a particular focus on macOS support.

Changes Made

• Platform-specific packaging
  • Unified package_data and depends handling with if / elif / else logic:
    • Windows → .dll
    • macOS → .dylib
    • Linux/others → .so
  • Backend libraries now follow the same pattern, ensuring .dylibs are copied into the package on macOS.
• Runtime linking improvements
  • macOS: Added Wl,-rpath,@loader_path so extensions can locate bundled .dylibs after installation.
  • Linux: Preserved $ORIGIN in runtime_library_dirs so .sos can be found next to the extensions.
  • Linux/macOS: Explicitly link against ffi from the active environment.
• Coverage builds
  • Fixed the typo MAC -> MACOS
• Backend Packaging Logic
  • Updated backend packaging logic to include macos by using if / elif / else for clarity.
• Project URL
  • Fixed incorrect GitHub URL (genn_team → genn-team).

Impact

• Fixes missing .dylib files on macOS installs.
• Ensures runtime linking works consistently across Windows, Linux, and macOS.
• Simplifies platform-specific logic for easier maintenance.
• Corrects metadata (URL) to point to the right repository.

[neworderofjamie]

Looks great - just a few minor formatting suggestions

[neworderofjamie]

The else clause above (starting at line 115) is basically LINUX or MACOS so I think it would be cleaner to put this logic in there rather than adding yet another if

[Agrim-P777]

yes updated it