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>

app/loop.go

@@ -10,8 +10,6 @@ import (
 	"gioui.org/app/internal/window"
 	"gioui.org/gpu"
 	"gioui.org/op"
-
-	_ "gioui.org/app/internal/d3d11"
 )
 
 type renderLoop struct {