cmake - fix error finding Eigen::Eigen target #7390

`EIgen::Eigen` target used in #7340 for linking exists only if there's a cmake config, while there are cases when there's just include directory. Added same name interface target to mimic config.

cmake/FindEigen3.cmake

@@ -0,0 +1,27 @@
+#
+# Input variables:
+# - `EIGEN_DIR`
+# If input variables are not specified, try to find Eigen3 config.
+# Input variables could also be provided as environment variables.
+#
+# Output targets:
+# - `Eigen3::Eigen`
+
+UNIFY_ENVVARS_AND_CACHE(EIGEN_DIR)
+
+if(EXISTS "${EIGEN_DIR}")
+    # Mimic Eigen3Config.cmake target.
+    add_library(Eigen3::Eigen INTERFACE IMPORTED)
+    target_include_directories(Eigen3::Eigen INTERFACE "${EIGEN_DIR}")
+else()
+    find_package(Eigen3 CONFIG)
+    if(Eigen3_DIR)
+        message(STATUS "Eigen3: found config at '${Eigen3_DIR}'.")
+    else()
+        message(
+            FATAL_ERROR
+            "EIGEN_DIR is not provided or provided folder doesn't exist (current value: '${EIGEN_DIR}'). "
+            "Also couldn't find Eigen3 as a package."
+        )
+    endif()
+endif()