Optional intermodule-dependencies are treated as required in top-level build

For example, it should be possible to do a top-level build of qtbase and qtdeclarative without qtsvg, because we have

dependencies:
  ../qtbase:
    ref: 7c572884608c87036b2e6cc214a2ee62c3b6100b
    required: true
  ../qtsvg:
    ref: a9c17def8ddab35b5d032d579f6569da006d9d14
    required: false

in qtdeclarative/dependencies.yaml

The reason is that we don't even read the required field and therefore treat all dependencies as required.

Note

When looking into this I noticed that cmake/TopologicalSort.cmake, which was originally used to top-sort the inter-module dependencies, is not used anymore - since we read dependencies.yaml instead of .gitmodules in fact.
When fixing this issue here, decide on the fate of cmake/TopologicalSort.cmake.