log.Panicf("no source in tree for import %q (from import %s in %s): %v", name, fromPath, fromDir, err)
	}

	context := w.context
	if context == nil {
		context = &build.Default
	}

	// Look in cache.
	// If we've already done an import with the same set
	// of relevant tags, reuse the result.
	var key string
	if usePkgCache {
		if tags, ok := pkgTags[dir]; ok {
			key = tagKey(dir, context, tags)