[C++] Fix CMake script to be used inside another project

The change to add support for exporting CMake config and target has introduced some minor issues when the ORC C++ library is used inside a larger CMake project. Mainly there are three issues:

1. We should prepend (not append) our CMake modules so we always use our own modules when there are naming conflict.
2. Do not use CMAKE_SOURCE_DIR and CMAKE_BINARY_DIR because they are tied to the root project and it is no longer ORC project when we are inside another project.
3. Add orc_ prefix to our CMake functions to avoid potential conflict.

See: https://github.com/apache/arrow/pull/45226