CMake package configuration name outdated

With Eigen 5.0.0 tagged it seems somewhat confusing to call the CMake package configuration Eigen3 and correspondingly the import target Eigen3::Eigen.

I suggest providing an additional version agnostic package configuration (i.e., EigenConfig.cmake) that exposes the Eigen::Eigen import target and deprecating the existing one with a scheduled removal in one of the next non-patch releases.

Renaming the package configuration to Eigen5 probably does not make much sense because it is unlikely that multiple Eigen versions will be used in the same project.