eigen3: Update Find Module

This MR is to trim down exposure of Eigen through an update to its Find Module. I've used the module from the Point Cloud Library available at:

https://github.com/PointCloudLibrary/pcl/blob/9dc4a0e1614cc955e36040c537753028f02a9feb/cmake/Modules/FindEigen.cmake

I documented it, and ported it to a proper imported library target.

This has the added benefit of reusing CMake's Find Module logic to handle versioning, and being able to remove all the dangling include_directories usages in favor of only the affected targets linking against it.

Test Plan

Build Krita.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.