Sync FindEigen3.cmake with the version from Eigen3 repo
Sync FindEigen3.cmake with the version from Eigen3 repo

Taken from https://gitlab.com/libeigen/eigen/-/blob/6fb3e5f1767855bc1a8aa3c868bc7fbf0eeb67ef/cmake/FindEigen3.cmake . 

As all the platforms that we support now install properly formed `Eigen3Config.cmake`, in any case this file will be removed in the next feature release of iDynTree. However, this fix make sure that iDynTree 1.0.* does not create problems with dependencies that require the Eigen3::Eigen target to be defined, see for example https://github.com/robotology/osqp-eigen/issues/61 .
robotology/idyntree,robotology/idyntree,robotology/idyntree,robotology/idyntree,robotology/idyntree