[vcpkg-tool] New field "conflicts" for vcpkg.json-manifest

[Agorath]

The vcpkg.json manifest needs a new field to specify whether a port conflicts with another port and cannot be built alongside it or vice versa.

Please review the discussion in #44023 to see why this issue arose.

To summarise, SDL3 is superseding SDL2. The team behind SDL has thus provided a drop-in replacement for SDL2 legacy applications, which uses SDL3 behind the scenes to make migration easier. As SDL2-compat is meant as a drop-in replacement for SDL2, its CMakeLists.txt places header files with the same name in the same directories and defines the library with the same name (SDL2::SDL2) as SDL2 does. Thus, the ports sdl2 and sdl2-compat can't be built next to each other.

One suggestion by @Cheney-W to achieve mutual exclusivity was to modify the portfiles of both ports to detect whether the other was also built, as has been done for other conflicting ports. But as @dg0yt pointed out in his comment this won't work in all cases.

I would thus like to suggest adding a new field, i.e. conflicts, to the vcpkg.json-manifest. It could look like the following:

{
    "name": "portZ",
    "version": "1.0.0",
    "dependencies": [],
    "conflicts": [
        "portX",
        "portY"
    ]
}

This way, a port can specify that it conflicts with another port, preventing conflicting ports from being built next to each other.

What do you think about this? Is this a valid approach for vcpkg or am I missing something?
If this approach is feasible, I would volunteer to try implementing this new feature and opening a pull request for vcpkg-tool when I'm done.

Looking forward to your feedback. 🙂