Move `third_party/xla_client` to `torch_xla/csrc/runtime` (#5160)
* Remove third_party/xla_client
* Move our `xla::` namespace to `torch_xla::runtime`
* Fix namespaces
* Skip `torch_xla::` in `torch_xla::` namespace
* Use shorthand for torch_xla/csrc/runtime
* Formatting
* FIx namespaces in C++ tests
* formatting
