onnxruntime upstream uses onnxruntime::onnxruntime: Adhere to that and remain compatible with our custom onnxruntime build

[davidrohr]

Upstream onnxruntime actually creates a CMake target, but with onnxruntime::onnxruntime not with ONNXRuntime::ONNXRuntime as does the hacked cmake recipe in our fork.

This commit changes O2 to use the upstream notation, and in case we use onnxruntime from our recipe not an upstream build, we create an alias to remain compatible for now.

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[davidrohr]

This seems to work as intended:

-- Could NOT find onnxruntime (missing: onnxruntime_DIR)
-- Found ONNXRuntime::ONNXRuntime: /sw/slc7_x86-64/ONNXRuntime/v1.12.1-alice1-local4/lib/libonnxruntime.so  

The upstream version with the normal onnxruntime naming is not found, but then our ALICE version is found: ONNXRuntime::ONNXRuntime

[alibuild]

Error while checking build/O2/fullCI for 9125714 at 2024-03-31 16:13:

## sw/BUILD/O2-latest/log
c++: error: unrecognized command-line option '--rtlib=compiler-rt'
c++: error: unrecognized command-line option '--rtlib=compiler-rt'

Full log here.