COMP: Expose itkeigen subdir in ITKEigen3_INCLUDE_DIRS for external consumers

ITK ships its vendored Eigen3 headers at
  Modules/ThirdParty/Eigen3/src/itkeigen/Eigen/<header>

ITK code accesses these via the ITK_EIGEN(<header>) macro
defined in itk_eigen.h, which expands to
  <itkeigen/Eigen/<header>>

When ITK_USE_SYSTEM_EIGEN=OFF, the prior ITKEigen3_INCLUDE_DIRS
contained only ${ITKEigen3_SOURCE_DIR}/src — sufficient for the
macro form (the compiler then appends /itkeigen/Eigen/<header>) but
not for the upstream Eigen convention <Eigen/<header>>, which needs
${ITKEigen3_SOURCE_DIR}/src/itkeigen on the include path.

External consumers that legitimately use the upstream pattern hit
'fatal error: Eigen/Dense: No such file or directory' as soon as
they link against ITK::ITKEigen3Module or ITK::eigen_internal.
This blocks the modern-target-interfaces refactor in
InsightSoftwareConsortium/ITKTotalVariation#57: proxTV's
lapackFunctionsWrap.cpp uses #include <Eigen/Dense> and so cannot
build against the vendored Eigen via either of ITK's exported
imported targets.

Add the itkeigen subdirectory to ITKEigen3_INCLUDE_DIRS so both
include conventions resolve.  ITK's own consumers continue to find
<itkeigen/Eigen/<header>> via the first entry; external consumers
now find <Eigen/<header>> via the second.  Both entries flow into
INTERFACE_INCLUDE_DIRECTORIES of the IMPORTED ITK::ITKEigen3Module
target, so the fix applies in-tree and to find_package(ITK)
consumers alike.

The Eigen3::Eigen IMPORTED target produced by Eigen's own export
already exposes the correct path, but it is in-source-scope only
(declared as add_library(Eigen3::Eigen ALIAS eigen)) and therefore
invisible to FetchContent sub-builds; relying on it is not an
option for downstream remote modules that pull proxTV via
FetchContent.

This change is independent of the Eigen 5 vendored update in
PR #6176; the architectural mismatch predates that PR and applies
to upstream/main as-is.

Author: hjmjohnson

Modules/ThirdParty/Eigen3/CMakeLists.txt

@@ -55,7 +55,18 @@ if(ITK_USE_SYSTEM_EIGEN)
   )
 else()
   set(ITKEigen3_LIBRARIES eigen_internal)
-  set(ITKEigen3_INCLUDE_DIRS ${ITKEigen3_SOURCE_DIR}/src)
+  # Two include directories are required so both ITK's own
+  # ITK_EIGEN(X) macro convention <itkeigen/Eigen/X> and the upstream
+  # Eigen convention <Eigen/X> resolve.  ITK code rooted at
+  # ${ITKEigen3_SOURCE_DIR}/src uses the macro form; external
+  # consumers (proxTV, anything else that includes Eigen the
+  # standard way) need the parent of <Eigen/X>, which is
+  # ${ITKEigen3_SOURCE_DIR}/src/itkeigen.
+  set(
+    ITKEigen3_INCLUDE_DIRS
+    ${ITKEigen3_SOURCE_DIR}/src
+    ${ITKEigen3_SOURCE_DIR}/src/itkeigen
+  )
 endif()
 
 # For the generated itk_eigen.h