Streamline dependencies built for tropter vs casadi

[carmichaelong]

Currently for building dependencies, even if OPENSIM_WITH_CASADI=ON and OPENSIM_WITH_TROPTER=OFF, eigen and colpack are still built (eigen and colpack are only needed for tropter)

https://github.com/opensim-org/opensim-core/blob/master/dependencies/CMakeLists.txt#L212

[aymanhab]

@carmichaelong I believe you fixed this already, please close or let me know otherwise.

[carmichaelong]

Reading back on the comments, I think the PR has the fix for the core build, but this issue should stay open for a fix for the dependencies build.