Use Eigen3::Eigen target

Since version 3.3 Eigen provides Eigen3Config.cmake with the imported
target Eigen3::Eigen. [1]
Use this imported target as descibed in Eigen-Wiki [2]

In the CeresConfig file improve relocatability by removing absolute
paths to the compiled dependencies. Instead find the used Eigen3::Eigen.
Furthermore use the find_dependency() [4] CMake function instead of the
find_package() call in CeresConfig.

This commit relies on all targets to be explicitly linked private or
public as done in the the change [3]

[1] https://bitbucket.org/eigen/eigen/pull-requests/257/cmake-imported-target-take-2/diff
[2] https://eigen.tuxfamily.org/dox/TopicCMakeGuide.html
[3] https://ceres-solver-review.googlesource.com/c/ceres-solver/+/16220
[4] https://cmake.org/cmake/help/latest/module/CMakeFindDependencyMacro.html

Change-Id: I44f44a089083f7169bcf430b59775242e4eb72d1