update dependency checks

[fbergmann]

I've noticed that the FindModules are not consistently used. Most of the checking for libraries / programs in the main CMakeLists.txt file should use the FindModules, so that the detection of dependencies is simplified (and we have an easier time to debug when and if things go wrong).

One example was the detection of ZLIB: https://github.com/sbmlteam/libsbml/pull/382