Use upstream expat, remove duplication in deps and src

Fix clash of builtin findEXPAT with config scripts

cmake/modules/FindEXPAT.cmake

@@ -0,0 +1,36 @@
+set(_q "")
+if(EXPAT_FIND_QUIETLY)
+    set(_q QUIET)
+endif()
+find_package(EXPAT ${EXPAT_FIND_VERSION} CONFIG ${_q})
+
+if(NOT EXPAT_FIND_QUIETLY)
+    if (NOT EXPAT_FOUND)
+        message(STATUS "Falling back to MODULE search for EXPAT...")
+    else()
+        message(STATUS "EXPAT found in ${EXPAT_DIR}")
+    endif()
+endif()
+
+if (NOT EXPAT_FOUND)
+    set(_modpath ${CMAKE_MODULE_PATH})
+    set(CMAKE_MODULE_PATH "")
+    include(FindEXPAT)
+    set(CMAKE_MODULE_PATH ${_modpath})
+
+    if (NOT TARGET EXPAT::EXPAT)
+        add_library(EXPAT::EXPAT INTERFACE)
+        target_link_libraries(EXPAT::EXPAT INTERFACE ${EXPAT_LIBRARIES})
+        target_include_directories(EXPAT::EXPAT INTERFACE ${EXPAT_INCLUDE_DIRS})
+    endif ()
+endif()
+
+if (EXPAT_FOUND AND NOT TARGET EXPAT::EXPAT)
+    add_library(libexpat INTERFACE)
+    add_library(EXPAT::EXPAT ALIAS libexpat)
+    target_link_libraries(libexpat INTERFACE expat::expat)
+    if (NOT EXPAT_LIBRARIES)
+        set(EXPAT_LIBRARIES expat::expat CACHE STRING "")
+    endif ()
+endif ()
+