gpu/internal/d3d11: make the Direct3D backend internal to package gpu

The package app/internal/d3d11 now contains only the GPU backend on
Direct3D. Move it below package gpu to reflect that.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/internal/d3d11/backend.go

@@ -0,0 +1,5 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+// This file exists so this package builds on non-Windows platforms.
+
+package d3d11