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/filetypes.go b/internal/filetypes/filetypes.go
index c03c1a8..4e291af 100644
--- a/internal/filetypes/filetypes.go
+++ b/internal/filetypes/filetypes.go
@@ -20,10 +20,10 @@
 	"path/filepath"
 	"strings"
 
+	"cuelang.org/go/cue"
 	"cuelang.org/go/cue/build"
 	"cuelang.org/go/cue/errors"
 	"cuelang.org/go/cue/token"
-	"cuelang.org/go/internal/legacy/cue"
 )
 
 // Mode indicate the base mode of operation and indicates a different set of