`bevy_shader` cleanups

ShaderReflectError has been deleted, as it was unused.

ShaderCache::new now accepts a RenderDevice, and ShaderCache::get does not. This is to reflect the fact that a ShaderCache must only be used with one RenderDevice for it to be valid.

The set_import_path, with_import_path, import_path, and imports methods on Shader have been removed. Just access the fields directly, these were superfluous getter methods.