Can I distribute an installer script which installs X, P and Q on the user's computer when they run it?
I have the feeling that you are treading on thin ice here and you should definitely consult a lawyer with experience in open-source licensing before betting a company on it.
Your safest option is to distribute X without any plugins and let the user ...
According to the GPL FAQ:
I would like to bundle GPLed software with some sort of installation software. Does that installer need to have a GPL-compatible license? (#GPLCompatInstaller)
No. The installer and the files it installs are separate works. As a result, the terms of the GPL do not apply to the installation software.
Copyright law is centered around making a copy of something and then doing things with that copy. How such a copy is made is usually not specified in the copyright laws, which makes that a "mental copy", where a work is read and then what was memorized is reproduced, can be regarded as a copy as well.
As copyright law works on actually making a ...