// needed to maintain any already-resolved versions of other modules.
// applyUpgrades does not mark the new versions as resolved, so they can still
// be further modified by other queries (such as wildcards).
//
// If all pathSets are resolved without any changes to the build list,
// applyUpgrades returns with changed=false.
func (r *resolver) applyUpgrades(ctx context.Context, upgrades []pathSet) (changed bool) {
	defer base.ExitIfErrors()