Allow including cmake files from project descriptor

Not sure if this is really required, but maybe allowing the user to include configuration directly into the `CMakeLists.txt` file could provide more flexibility. One solution could be to allow including cmake configuration files, so that no weird syntax is required. One could go from allowing the inclusion of cmake contents before, after or mid sections in the `CMakeLists.txt`.

Start by tracing how the project descriptor produces the CMakeLists.txt file and where its sections are assembled. Determine whether including cmake configuration before, after, or between sections is supported by the current design. Done means the intended include positions and configuration format are specified and covered by tests.

Written by the indexing model from the issue text.