How do I find the reverse dependency of a package?

You can use apt-cache rdepends --installed jetty to see what depends on jetty. This will show both depends & recommends, so you may want to check through the list of packages to see what the relationship is with jetty.

Go install apt-rdepends. It doesn't only show immediate dependencies, but all the recursive ones, so will tend to give you a large output. So, use a pager as well.

• To display recursive depends: $ apt-rdepends jetty | pager.
• To display recursive reverse depends: $ apt-rdepends --reverse jetty | pager.

If you open Synaptic and find the package, you can right click on it and select Properties. Under the tab Dependencies you can see the packages that jetty depend on, and which packages is dependent on jetty (the dropdown list).