cue/load: use error registry in SkipImports mode

Using the nil registry caused the module file to be
read in legacy mode which didn't populate the version,
which we need in the next CL.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: If20ef63b483bec39642eb0aeafa545ef775d4c2f
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1217213
Reviewed-by: Marcel van Lohuizen <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: rogpeppe

cue/load/config.go

@@ -368,8 +368,8 @@ func (c Config) complete() (cfg *Config, err error) {
 	}
 	if c.SkipImports {
 		// We should never use the registry in SkipImports mode
-		// but nil it out to be sure.
-		c.Registry = nil
+		// but make it always return an error just to be sure.
+		c.Registry = errorRegistry{errors.New("unexpected use of registry in SkipImports mode")}
 	} else if c.Registry == nil {
 		registry, err := modconfig.NewRegistry(&modconfig.Config{
 			Env: c.Env,