[pybind] Improve pip package build

[pytorchbot]

Addressing comments in #5961.

• Separate out executorch-wheel-config.cmake from executorch-config.cmake.
• Hardcode the envrionment variable SETUPTOOLS_EXT_SUFFIX in setup.py.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/6142

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❌ 14 New Failures

As of commit cc2cde8 with merge base eecf74f ():

NEW FAILURES - The following jobs have failed:

• pull / test-binary-size-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 31533f43132d01c4293e82991ab030d85220b30c0a7f4d657be40836dec8a864 /exec failed with exit code 2
• pull / test-binary-size-linux-gcc / linux-job (gh)
  RuntimeError: Command docker exec -t 771e23d79136fc8f51e799d50c0a87d36874c3afed76e5f7097d218ca551e21f /exec failed with exit code 2
• pull / test-custom-ops-linux (buck2) / linux-job (gh)
  RuntimeError: Command docker exec -t 0ac77c8d8e02560a10db7be6a2e9e4dc4a2280698922af29baa7594db5430e4a /exec failed with exit code 2
• pull / test-custom-ops-linux (cmake) / linux-job (gh)
  RuntimeError: Command docker exec -t 8cda3d17edfaf5c88e792493de3daeb15934ec9fe5623080bfe186b110bc0542 /exec failed with exit code 2
• pull / test-llama-runner-linux (fp32, cmake, portable) / linux-job (gh)
  RuntimeError: Command docker exec -t 1128734ce5ddd6655c160027caa4410cc619923a26e4341673d181d2f536b05f /exec failed with exit code 2
• pull / test-llama-runner-linux (fp32, cmake, xnnpack+custom) / linux-job (gh)
  RuntimeError: Command docker exec -t 0b88b29e04b783bff8f6b8c113378c382eb270d10a8a4b17185d4502b029d9aa /exec failed with exit code 2
• pull / test-llama-runner-linux (fp32, cmake, xnnpack+custom+qe) / linux-job (gh)
  RuntimeError: Command docker exec -t f68fb687dfe9daa463f65d603747f1427197c29a0eda207e1ea9f329ee11ecdc /exec failed with exit code 2
• pull / test-llama-runner-linux-android (cmake) / linux-job (gh)
  RuntimeError: Command docker exec -t 56cee457c9e0ef76e00778b90d57c4bda577e3fc0fed4acb5b88306369fadef2 /exec failed with exit code 2
• pull / test-llama-runner-qnn-linux (fp32, cmake, qnn) / linux-job (gh)
  RuntimeError: Command docker exec -t e829b2cfa896689e3e88939eece30435f215f259a07e25caf34291141ef8afb3 /exec failed with exit code 2
• pull / test-llava-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 75619412e73ee36114dbb77029c88c005fabcb2a53792475142f1935483e0607 /exec failed with exit code 2
• pull / test-selective-build-linux (cmake) / linux-job (gh)
  RuntimeError: Command docker exec -t a222e5760848326ce50d65a0341354cc2165aadd2b5e5cc214ea5b6c750c2c01 /exec failed with exit code 2
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 013a3580b7d4d9274e97fc5041bba1e42af733c7706b8868bf0c4bdadd1ecd78 /exec failed with exit code 2
• pull / unittest / macos / macos-job (gh)
  Library not loaded: @rpath/liblz4.1.dylib
• pull / unittest-arm (buck2) / linux-job (gh)
  ../../dev/ops/test_add.py::TestSimpleAdd::test_add2_tosa_BI_4

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