Consider removing headless plugins

[alice-i-cecile]

          I'm not sure this is a good idea to have in Bevy. "Headless" meaning will change depending on the use case, and controlling it with a plugin rather than features means longer compile time and a lot of code that can't be used, events not registered, component not reflected, but still available when developing which will mean a less good developer experience.

Originally posted by @mockersf in #15260 (comment)

[alice-i-cecile]

I'm sympathetic to this perspective, but I'm really unhappy about how complex it is for users to solve a common request: "please just run my app headless for a server or CI" state. Without this type, controlling this with plugin disabling and/or feature flags currently requires pretty arcane knowledge of the every plugin and feature flag in Bevy.

[mockersf]

we could add a feature group for headless that would enable the feature flags one would expect for that and call those "feature profiles"

We could even make ones for "basic 2d", "basic 3d" and "headless" that would just be subset of the default features

some day, the editor will expose something like this screen from Godot:

As an example, here is the list of features I tend to use for 2d games:

    "bevy_asset",
    "bevy_audio",
    "bevy_core_pipeline",
    "bevy_render",
    "bevy_sprite",
    "bevy_state",
    "bevy_text",
    "bevy_ui",
    "bevy_winit",
    "multi_threaded",
    "vorbis",
    "png",
    "webgl2",
    "x11",

[alice-i-cecile]

Sounds great: I'll tackle this.

[heydocode]

Excellent issue, I'm not using headless plugins but it will be great if the compile-time of CI/server side projects will compile faster and will have less bloat in the final binaries (that also concerns games with visuals, please track your dependencies & its features, guys). In the current state of rust, its linker cannot predict with absolute precision which dependencies won't be used at runtime, so I think only 85% of the final binary will contain the code you really need if you import too heavy dependencies without setting the only features you need.