wire: use package names to disambiguate variables (google/go-cloud#386)

internal/wire/testdata/NamingWorstCase/want/wire_gen.go

@@ -12,9 +12,9 @@ import (
 // Injectors from wire.go:
 
 func inject(context3 context2.Context, err2 struct{}) (context, error) {
-	context4, err := provide(context3)
+	mainContext, err := provide(context3)
 	if err != nil {
 		return context{}, err
 	}
-	return context4, nil
+	return mainContext, nil
 }