Prior to this patch, the ONNXConfig.cmake script (generated from the
ONNXConfig.cmake.in file) tries to read the export target file
(ONNXTargets.cmake) from the same directory where ONNXConfig.cmake is
located, but the two files are not in the same directory.  Specifically,
CMake places the export target file (ONNXTargets.cmake) in a directory
whose name contains the build hash, whereas the ONNXConfig.cmake script
is placed in the ${PROJECT_BINARY_DIR}/ONNXConfig.cmake, as specified in
the top-level CMakeLists.txt directory.

This patch reconciles the difference by explicitly instructing CMake to
place the export target file (ONNXTargets.cmake) in the same directory
as the ONNXConfig.cmake script during the build step.

This patch also makes two ancillary changes.  First, it adds the
`ONNX::` namespace to the target names, so that targets built by this
project don't interfere with targets built by other projects.  Second,
this patch adds Protobuf to the list of packages that consumers of ONNX
must have available, since protobuf libraries are linked publicly while
building ONNX.

Signed-off-by: Ashay Rane <ashay@users.noreply.github.com>