of Go values and free that space manually all at once, safely. The purpose
of this functionality is to improve efficiency: manually freeing memory
before a garbage collection delays that cycle. Less frequent cycles means
the CPU cost of the garbage collector is incurred less frequently.

This functionality in this package is mostly captured in the Arena type.

	name := fromPath
	if canonical, ok := w.importMap[fromDir][fromPath]; ok {
		name = canonical
	}

	pkg := w.imported[name]
	if pkg != nil {
		if pkg == &importing {
			log.Fatalf("cycle importing package %q", name)
		}
		return pkg, nil
	}
	w.imported[name] = &importing

	// Determine package files.
	dir := w.importDir[name]
	if dir == "" {