cue: make internal cue port main API implementation

- rm cue/*.go
- mv internal/legacy/cue/*.go cue/
- rename imports
- go

Change-Id: I553ce372ad457b1d73126b0a3f7be8224e415e80
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6742
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/internal/filetypes/gen.go b/internal/filetypes/gen.go
index 6430ff9..f0b9f03 100644
--- a/internal/filetypes/gen.go
+++ b/internal/filetypes/gen.go
@@ -21,9 +21,9 @@
 	"log"
 	"os"
 
+	"cuelang.org/go/cue"
 	"cuelang.org/go/cue/load"
 	"cuelang.org/go/encoding/gocode"
-	"cuelang.org/go/internal/legacy/cue"
 )
 
 func main() {