Always emit json metadata, and emit entry point names

[khyperia]

Fixes #620

[VZout]

Awesome! and looks quite easy to add interface variables as well to the metadata. But this does not work with multimodule?

[khyperia]

But this does not work with multimodule?

yeah, it does work? cargo run -p multibuilder outputs this:

CompileResult {
    module: MultiModule(
        {
            "main_fs": "/home/khyperia/me/rust-gpu/target/spirv-unknown-spv1.3/release/deps/sky_shader.spv.dir/main_fs",
            "main_vs": "/home/khyperia/me/rust-gpu/target/spirv-unknown-spv1.3/release/deps/sky_shader.spv.dir/main_vs",
        },
    ),
    entry_points: [
        "main_fs",
        "main_vs",
    ],
}

the code does take a little shortcut and yoinks the entrypoint out of the precomputed dictionary (which itself yoinks the entrypoint out of the module) instead of digging around in the module itself, because it's a bit simpler code, but it could easily be expanded to dig through the modules themselves if we wanted to extract more metadata (or carry information through the compiler and emit it, instead of reflecting the output)

[VZout]

Ah I saw this and thought metadata was disabled in its entirety for multimodule.

if self.print_metadata && self.multimodule {
    return Err(SpirvBuilderError::MultiModuleWithPrintMetadata);
}

[khyperia]

Ah, that's printing cargo metadata, and is no change in behavior from the current system - it already produces the same error before this PR. "Metadata" in the context of that code means things like printing cargo:rerun-if-changed=blah to stdout.