Relax dependencies of external projects examples on all qt_plugins in a top-level build

Currently when configuring a top-level build with examples as external projects (in-tree), trying to build an example will try to build all qt_plugins which will build a big chunk of qt (including webengine).

We add a dependency on all qt plugins for safety, because we don't have the information on which the plugins examples depend on.

But generally examples shouldn't depend on plugins outside of the repo they are defined in.

So we can relax the dependencies so that qtbase examples depend only on qtbase plugins, qtdeclarative examples on qtdeclarative plugins and plugins of its dependent repos, etc.