pkg/types: introduce a URLs type

Cleanup the usage of URLs into its own type so we don't have to use a
FlagValue everywhere we have a list of URLs.
i think we could just use
```
+func (us URLs) Len() int { return len(us) }
+func (us URLs) Less(i, j int) bool { return us[i].String() < us[j].String() }
+func (us URLs) Swap(i, j int) { us[i], us[j] = us[j], us[i] }
```