Don't include prerequisites here; do it more explicitly

Developers that are working to familiarize themselves with OPM will
probably start out in CMakeLists.txt to figure out where the project
declare its dependencies, thus the list should be referred to from
there instead of inside one of the sub-modules.
Include file list from top-level instead of module-level

Increase the visibility of the source file name include by no longer
putting it in a module, but rather by including it in CMakeLists.txt
Refer to other project-specific files at top-level

Having the include statements here will hopefully lead to others
quickly finding the list of prerequisite files needed to build.