Revisit pkglinks construction

[AMDmi3]

It turns out that we need to update packagelinks as soon as we update name mappings (#944), which is inconvenient and error prone. So the idea is to use source name IDs in the links. We can't implement it right now as links are constructed on the fly from the package information where we don't have information on source names, however it may be easily implemented as soon as we have #722 and links are constructed during repository parsing.