Glad you've found a working solution! To make it easier for people being afraid of working with the terminal:
On the Google Playstore you can find the app SD Maid, which (amongst other things) can take care for "left-overs" of uninstalled apps, which it calls "corpses" (so the part relating to this is the "corpses finder"). To do so, it reads the list of installed apps from the package manager, and then checks for e.g. directories which should not be there:
SDMaid: "Corpse Finder" and "System Cleaner" (Source: Google Play; click images to enlarge)
In your case, you've uninstalled the "foobar" app, which has the "package-name" of "com.foobar.app". Reading the list of installed apps, SDMaid thus does not find it -- so the folder
/data/data/com.foobar.app would be identified as "corpse" and offered for cleaning.
So the big pro for the "average user" here is not only that he can avoid the command line, but also does not need to figure out tha apps "technical name", as SDMaid does that itself. And as "nice-to-have", the app also brings a file manager, file searcher, app control (freezing apps etc), and more.
Last remark: your device must be rooted for SDMaid to work. But so it must if you want to manually remove that directory, as the
su step in your own answer shows :)