How can I see all installed packages, including those that have been hidden?
If you run adb shell pm you would find many options and parameters for pm. One of the parameter is -u to show uninstalled packages. (Hidden packages are treated as uninstalled packages.)

On an unrooted device, make a list of all the packages and then all the packages plus the hidden ones. Compare them and the uncommon entries in the second list would be the packages you are looking for. 

adb shell pm list packages
adb shell pm list packages -u


As for rooted devices, search the string hidden="true" in the file at /data/system/users/0/packages-restrictions.xml. You would see the corresponding package name. 

Tested with Stock Android 6.0.1.