cue: move to external builtin packages

This includes a new generator for builtin packages.

Change-Id: Ieab84e16d9136c2ad5d2d8e8cad49c84a6e0b658
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6885
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
diff --git a/internal/filetypes/types.go b/internal/filetypes/types.go
index 4c47c9b..33efc31 100644
--- a/internal/filetypes/types.go
+++ b/internal/filetypes/types.go
@@ -7,6 +7,7 @@
 
 	"cuelang.org/go/cue"
 	"cuelang.org/go/encoding/gocode/gocodec"
+	_ "cuelang.org/go/pkg"
 )
 
 var cuegenCodec, cuegenInstance = func() (*gocodec.Codec, *cue.Instance) {