deps/eigen3: Use Eigen 3.3.9 from source only on Ubuntu 18, use apt/eigen on newer operating systems

• Rename module deps/eigen3 to deps/eigen3/3.3.9,
• Add new module deps/eigen3 redirecting to deps/eigen3.3.9 on bionic, to apt/eigen3 else
• Update usages of apt/eigen3 to deps/eigen3

Related:

• #242 (closed)
• Simox: sw/simox/simox#66 | sw/simox/simox!117 (merged)
• MMM Tools: sw/mmm/tools!10 (merged)
• RobotAPI: sw/armarx/robot-api!306 (merged)

See especially sw/simox/simox!117 (comment 39222)

Tested on:

• Updated: Compilation on Ubuntu 18
  • To switch from apt/eigen to deps/eigen/3.3.9, clearing the build directories appears to be necessary
• Updated: Compilation on Ubuntu 20, deps/eigen deleted, with sw/simox/simox!117 (merged)