cue/load: do not format registry as a string

I saw an error message like this:

```
import failed: cannot get directory for external module "github.com/cue-labs-modules-testing" (registry &{%!q...): no dependency found for import path "github.com/cue-labs-modules-testing"
```

which is clearly wrong. Unfortunately `go vet` doesn't complain when `%q` is applied to interface arguments.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I2ae9fb9765da6274dd17ef643a9e0aca75a3a03d
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1171754
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cue/load/import.go

@@ -394,7 +394,7 @@ func (l *loader) absDirFromImportPath(pos token.Pos, p importPath) (absDir, name
 			absDir, err = l.externalPackageDir(p)
 			if err != nil {
 				// TODO why can't we use %w ?
-				return "", name, errors.Newf(token.NoPos, "cannot get directory for external module %q (registry %q): %v", p, l.cfg.Registry, err)
+				return "", name, errors.Newf(token.NoPos, "cannot get directory for external module %q: %v", p, err)
 			}
 		} else {
 			absDir = filepath.Join(GenPath(l.cfg.ModuleRoot), sub)