Support building modules for both branch-groups from single configuration
Currently kdesrc-build supports three branch groups
- stable-kf5-qt5: Stable branches for everything; based on Qt5
- kf5-qt5: Development branches and/or last branches with Qt5 support. kf5 for Frameworks, Plasma/5.27 for Plasma, master for most apps
- kf6-qt6: Branches for KF6 development. master for Frameworks and Plasma, master or kf6 for apps
This global approach to selecting the Qt/KF5 version doesn't match reality. Some projects like Plasma exclusively support Qt6 in their development version, others like e.g. Okular only support Qt5, and most apps support both (either transitionally like Kate, or permanently for the forseeable future like kio-extras), sometimes with the Qt6 support in a separate branch. Some projects (e.g. plasma-integration) even support both in a single build.
Having users specify a Qt/KF version is also approaching things in the wrong way. People don't care about Qt versions, they (usually) just want to build the latest development version of whatever they are interested in. For Plasma that means using kf6-qt6, for Okular that means kf5-qt5. So if you want to hack on both you'd need separate kdesrc-build setups.
Ideally a single kdesrc-build setup would suffice to build a full session of everything in KDE, regardless of whether the app is based on Qt5 or Qt6. That means that sometimes things will need to be build twice, like a build of Frameworks for kf5 and for master, and some Plasma components also need to be build for 5 and for 6. This only works if those modules are coinstallable, but we need that to be the case anyway