func normalizeAbsPath(path string) string {
	u, err := url.Parse(path)
	if err != nil {
		debugLog("normalize absolute path failed: %s", err)
		return path
	}
	return u.String()
}
normalize absolute path for cache.
on Windows, drive letters should be converted to lower as scheme in net/url.URL
package spec
import (
	"fmt"
	"net/url"
	"os"
	"path"
	"path/filepath"
	"strings"
)