"But is referred to by another package." - Finding that package

Sometimes this happens because APT just doesn't know anything about what you're talking about, but that can be solved by running:

sudo apt-get update

After running this, typically the error goes away because apt and apt-get know how to do what I ask now.

You can search for the package with apt-cache:

apt-cache search x

This will output all packages that in a way or another make a reference to x.