Introduce hierarchical platform flavor configuration

This is motivated by some of the Qt 6 CI builds needing different dependency or unit test options, but we are seeing similar needs in the static or custom build configurations as well, so better solve this sufficiently generically.

This extends the simple OS-only platform string to a hierarchical platform specifier, for backward compatibility we assume the first element is still the OS. The main feature however is that this is partially matched against configurations, ie. "Linux" applies to e.g. "Linux/Qt6" and "Linux/Qt5/static".