Reexport items needed to create a ShaderSource::Glsl

[yanchith]

Is your feature request related to a problem? Please describe.

I recently removed shaderc from my game's engine and started relying on naga to compile glsl in runtime (for both hot-reloading and not). All shaders work great, but the experience of doing this was somewhat sketchy.

ShaderSource::Glsl requires you to provide a naga::ShaderStage and a rustc_hash::FxHashMap, neither of which is exported from wgpu, unless I am mistaken. So I had to add top-level dependencies on these that version-match with what the gfx ecosystem uses.

Describe the solution you'd like
I'd like to import these directly from wgpu, something like:

let foo = wgpu::ShaderSource::Glsl {
    shader: Cow::from(FOO_VS),
    stage: wgpu::naga::ShaderStage::Vertex,
    defines: wgpu::naga::FastHashMap::default(),
}