Valve recently implemented functionality to automate some frequently asked Steam support questions such as permanently hiding a game from your Steam account and these package names being exposed are a result of them not hiding them when they added this. A similar issue occurred with app names as well.
Due to the way Steam works, we are now also able to guess some app names that were previously unknown based on what packages they are in. An example of this can be found here, where the 3 packages it is connected to clearly indicate what the name of the app would be. We ran a script that detected these kinds of apps and added their name to the "Last Known Name" field in the table on the top. This can also be seen in the app history with the source being "Best guess based on help website disclosure".
While this information comes straight from Valve and many of these kinds of leaks turned out to be real in the past, it is important to note that third-party Steam developers can create and name their own apps/packages, a noteworthy mention of developers doing this are these packages, in which they are trying to XSS Steam. (EDIT: It looks like the XSS packages might have been by Valve themselves. Thanks for letting us know, aiusepsi!)
Many things could also be old entries and as such no longer be in development/coming to Steam. An example of a previous leak that ended up being nothing (yet) is Halo 3.
TLDR: Half-Life 3 still not confirmed.