Allow opening single modules in qt repos as standalone projects

From Qt Contributor summit 2023 https://wiki.qt.io/Build_system_improvements_and_feedback

We got feedback that opening the qtbase project in Qt Creator is painful, it takes a long time to configure and then to index (code model) all sources.
And also that git pull-ing usually rebuilds too much, likely because bootstrap sources are touched, and therefore rebuilds moc and everything else.

The request was to be able to open just a specific qt module (qt core, qt gui, etc), in the hopes that this provides a better experience.

This should be doable in theory, if we modify each module cmakelists.txt to contain the usual project() + cmake_minimum_required + find_package() stanzas, like we do for the latest generation of standalone auto tests.

But there are still some things to consider:
how to find tools (likely need to configure with a QT_HOST_PATH)
is there a way to not specify all find_package() dependencies
what exactly should happen during build (in a no-prefix build) and install, should it override existing files
how to handle features (especially the global features that are tied to qtcore)
how to provide a nicer UX for this in Qt Creator (people won't want to specify QT_HOST_PATH, they will expect things to just work)
probably other stuff