In my opinion.
1. If you have all the components of the package, i.e.: base game + DLC, a new package consisting of these components in the library should have the "in library" tag
2. If you buy a package consisting of a game and DLC, these components in the library as separate items should also have the "in library" tag
I could never understand the twisted policy of interpreting the same packaged in a different way as a different product. You enter a series of your favorite game and you don't have half of them on the list. But... no, you have everything - once you conduct a thorough investigation and find the components. Why make life so difficult for players?