BUG: Restore eigen_internal export to ITK's main targets set (#6239)

Commit 64ddc66 (Eigen 5.0.1 vendored update, 2026-05-01) silently
moved eigen_internal out of ITK's main `${ITK3P_INSTALL_EXPORT_NAME}`
export set into a standalone `ITKInternalEigen3Targets`. Because
`ITKConfig.cmake` only loads `ITKTargets.cmake`, downstream
`find_package(ITK)` could no longer resolve `eigen_internal` and any
consumer using FetchContent with `FIND_PACKAGE_ARGS` (SimpleITK's
pattern) hit a target-collision cascade as CMake fell back to
`add_subdirectory`.

Re-add the install rule that registers `eigen_internal` in
`${ITK3P_INSTALL_EXPORT_NAME}` while keeping the existing standalone
`ITKInternalEigen3Targets` install for `find_package(ITKInternalEigen3)`
consumers. Both export sets now reference the same target.

Issue: #6239

Author: hjmjohnson

Modules/ThirdParty/Eigen3/src/itkeigen/CMakeLists.txt

@@ -965,6 +965,12 @@ target_include_directories (eigen_internal SYSTEM INTERFACE
 
 # Export as title case Eigen
 set_target_properties (eigen_internal PROPERTIES EXPORT_NAME Eigen)
+# Register in ITK's main export set so find_package(ITK) resolves
+# eigen_internal as an IMPORTED target loaded from ITKTargets.cmake
+# (issue #6239: SimpleITK FetchContent + FIND_PACKAGE_ARGS fallback).
+install (TARGETS eigen_internal EXPORT ${ITK3P_INSTALL_EXPORT_NAME})
+# Also register in the standalone ITKInternalEigen3 export set used by
+# find_package(ITKInternalEigen3) consumers.
 install (TARGETS eigen_internal EXPORT ITKInternalEigen3Targets)
 
 set(EIGEN3_TARGETS_FILE ITKInternalEigen3Targets.cmake)