COMP: Fix ExternalData when ITK is built with FetchContent

Addresses two issue:
- When ITK is built from FetchContent, the fetched source is outside of
the main source. This is an error with external data paths, so use the
ITK PROJECT source path as the default for ITK's External data.
- Use PROJECT_BINARY_DIR for output of download data, to prevent
target conflics with the main project.

Author: blowekamp

CMake/ExternalData.cmake

@@ -609,7 +609,7 @@ function(_ExternalData_arg target arg options var_file)
 
   # Convert to relative path under the source tree.
   if(NOT ExternalData_SOURCE_ROOT)
-    set(ExternalData_SOURCE_ROOT "${CMAKE_SOURCE_DIR}")
+    set(ExternalData_SOURCE_ROOT "${PROJECT_SOURCE_DIR}")
   endif()
   set(top_src "${ExternalData_SOURCE_ROOT}")
   file(RELATIVE_PATH reldata "${top_src}" "${absdata}")

CMake/ITKExternalData.cmake

@@ -28,10 +28,10 @@ list(
   APPEND
   ExternalData_OBJECT_STORES
   # Local data store populated by the ITK pre-commit hook
-  "${CMAKE_SOURCE_DIR}/.ExternalData"
+  "${PROJECT_SOURCE_DIR}/.ExternalData"
 )
 
-set(ExternalData_BINARY_ROOT ${CMAKE_BINARY_DIR}/ExternalData)
+set(ExternalData_BINARY_ROOT ${PROJECT_BINARY_DIR}/ExternalData)
 
 # Expands %(algo:lower)
 set(ExternalData_URL_ALGO_CID_lower cid)