Reorganize CPack management
- Move all CPack variable setting to a separate file to reduce clutter
in the main CMakeLists.txt.
- Move calling CPack to the end of the build system, since this allows
gathering information for CPack from other parts. Also some other
CPack features (like identifying the existing components) works better
there.
- Instead of hard-coding the generated directories to include in the
source package, add a helper function that can be called from the code
that actually does the generation. This makes it a lot easier to
understand how this works from just reading the CMake code.
- Add some TODOs for further improvements.
Change-Id: I2ca76f0f3b3e848527f2340b4aab1d62a5de8328