Refactoring: use checksums instead of MD5 for pool/published

This is related to #506

As a first step, don't pass MD5 explicitly, pass checksum info object,
so that as a next step we can choose which hash to use.

There should be no functional changes so far.

Next step: stop returning explicit paths from public package pool.

cmd/package_show.go

@@ -88,7 +88,7 @@ func aptlyPackageShow(cmd *commander.Command, args []string) error {
 		if withFiles {
 			fmt.Printf("Files in the pool:\n")
 			for _, f := range p.Files() {
-				path, err := context.PackagePool().Path(f.Filename, f.Checksums.MD5)
+				path, err := context.PackagePool().Path(f.Filename, f.Checksums)
 				if err != nil {
 					return err
 				}