cmd/cue: use parser.PackageClauseOnly in 'get go'

When importing CUE files, we parse each file to obtain its package name.
We only need to parse up to the package clause for this to work.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I21247bf16fed1cc3985bd1ef3f98a5be62ceb425
Dispatch-Trailer: {"type":"trybot","CL":1200969,"patchset":3,"ref":"refs/changes/69/1200969/3","targetBranch":"master"}

cmd/cue/cmd/get_go.go

@@ -597,7 +597,7 @@ func (e *extractor) importCUEFiles(p *packages.Package, dstDir, args string) err
 			if !ok {
 				return nil
 			}
-			f, err := parser.ParseFile(path, nil)
+			f, err := parser.ParseFile(path, nil, parser.PackageClauseOnly)
 			if err != nil {
 				return err
 			}