Don't build all of extragear and playground by default

This module-set block was added to default BUILD_WITH_QT6 to ON for any of the specified module sub-tree in kde-projects.

However, using module-set ends up pulling all of the modules in those sub-trees into the active build set, building many dozens of modules that cannot actually be built with Qt/K6 yet.

Commit fa11f2ee recently added a better way of doing this to kdesrc-build, which is to just add an options block. This is also moved up here, allowing the files included after to further override the options in other blocks.