Error Unknown Module(s) in QT: plugin

[-johnf-]

I'm running QT on a Raspberry Pi4 with Buster. I used apt-get to install the official release of QT 5.11.3. I can create an application, compile and run without issues. That is until I add "plugin" to my .pro file on the QT tag. Immediately after saving and closing the .pro file, I get the message ERROR Unknown Module(s) in QT: plugin. Obviously there is a missing library. I searched the repository for packages that may apply, but didn't see anything that looked relevant. All of the designer packages that I tried were already installed. It would nice if error messages like this would actually say what modules were missing, but they don't. So my question is: what libraries or packages are needed for the plugin subsystem to work? I'd really like to not compile Qt from source. I did have a pre-compiled Qt 5.12 LTS on the system where the plugin module worked. It didn't have support for Bluetooth though which I need. Is plugin support something that needs to be configured into the build like bluetooth?

Added after 1 28 minutes:

I installed qttools5-dev, qttools5-dev-tools and qttools5-private-dev. This resolved the problem.