Avoid overwriting the lightning_app entry point

[awaelchli]

What does this PR do?

If both lightning and lightning-app is installed, and in particular lightning is installed after lightning-app, then the entry point of the main lightning overwrites the lightning_app entry point. After that, running an app with lightning_app run app app.py is no longer possible, resulting in this error:

Traceback (most recent call last):
  File "/opt/lightning/envs/main/bin/lightning_app", line 8, in <module>
    sys.exit(_cli_entry_point())
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/lightning/__init__.py", line 49, in _cli_entry_point
    main()
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/lightning/app/cli/lightning_cli.py", line 120, in main
    _main()
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/lightning/app/utilities/exceptions.py", line 37, in invoke
    return super().invoke(ctx)
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/lightning/app/cli/lightning_cli.py", line 449, in run_work
    run_lightning_work(
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/lightning/app/launcher/utils.py", line 42, in wrapper
    res = func(*args, **kwargs)
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/lightning/app/launcher/utils.py", line 67, in wrapper
    res = func(*args, **kwargs)
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/lightning/app/launcher/launcher.py", line 149, in run_lightning_work
   load_app_from_file(file)
  File "/opt/lightning/envs/main/lib/python3.10/site-packages/lightning/app/utilities/load_app.py", line 131, in load_app_from_file
    raise MisconfigurationException(
lightning.app.utilities.exceptions.MisconfigurationException: The provided file /content/app.py does not contain a LightningApp. Instantiate your app at the module level like so: `app = LightningApp(flow, ...)`

This PR avoids that by simply avoiding the redirection.

This was originally introduced in #19440 but I don't think it was intentional, and the removal should not be in conflict with that PR. We have redirection logic for the sdk anyway.

---

📚 Documentation preview 📚: https://pytorch-lightning--20041.org.readthedocs.build/en/20041/

cc @Borda @carmocca

[github-actions]

🌩️ Required checks status: Pending 🟡

Groups summary

🟢 pytorch_lightning: Azure GPU

Check ID	Status
pytorch-lightning (GPUs) (testing Lightning | latest)	success	✅
pytorch-lightning (GPUs) (testing PyTorch | latest)	success	✅

These checks are required after the changes to src/lightning/__setup__.py.

🟢 lightning_fabric: Azure GPU

Check ID	Status
lightning-fabric (GPUs) (testing Fabric | latest)	success	✅
lightning-fabric (GPUs) (testing Lightning | latest)	success	✅

These checks are required after the changes to src/lightning/__setup__.py.

🟡 lightning: Store

Check ID	Status
store-cpu (macOS-11, lightning, 3.10, 2.0)	queued	⌛
store-cpu (ubuntu-20.04, lightning, 3.10, 2.0)	success	✅
store-cpu (windows-2022, lightning, 3.10, 2.0)	success	✅

These checks are required after the changes to src/lightning/__setup__.py.

🟢 mypy

Check ID	Status
mypy	success	✅

These checks are required after the changes to src/lightning/__setup__.py, src/version.info.

🟡 install

Check ID	Status
install-pkg (ubuntu-22.04, app, 3.8)	no_status	❓
install-pkg (ubuntu-22.04, app, 3.11)	no_status	❓
install-pkg (ubuntu-22.04, fabric, 3.8)	no_status	❓
install-pkg (ubuntu-22.04, fabric, 3.11)	no_status	❓
install-pkg (ubuntu-22.04, pytorch, 3.8)	no_status	❓
install-pkg (ubuntu-22.04, pytorch, 3.11)	no_status	❓
install-pkg (ubuntu-22.04, lightning, 3.8)	no_status	❓
install-pkg (ubuntu-22.04, lightning, 3.11)	no_status	❓
install-pkg (ubuntu-22.04, notset, 3.8)	no_status	❓
install-pkg (ubuntu-22.04, notset, 3.11)	no_status	❓
install-pkg (macOS-12, app, 3.8)	no_status	❓
install-pkg (macOS-12, app, 3.11)	no_status	❓
install-pkg (macOS-12, fabric, 3.8)	no_status	❓
install-pkg (macOS-12, fabric, 3.11)	no_status	❓
install-pkg (macOS-12, pytorch, 3.8)	no_status	❓
install-pkg (macOS-12, pytorch, 3.11)	no_status	❓
install-pkg (macOS-12, lightning, 3.8)	no_status	❓
install-pkg (macOS-12, lightning, 3.11)	no_status	❓
install-pkg (macOS-12, notset, 3.8)	no_status	❓
install-pkg (macOS-12, notset, 3.11)	no_status	❓
install-pkg (windows-2022, app, 3.8)	no_status	❓
install-pkg (windows-2022, app, 3.11)	no_status	❓
install-pkg (windows-2022, fabric, 3.8)	no_status	❓
install-pkg (windows-2022, fabric, 3.11)	no_status	❓
install-pkg (windows-2022, pytorch, 3.8)	no_status	❓
install-pkg (windows-2022, pytorch, 3.11)	no_status	❓
install-pkg (windows-2022, lightning, 3.8)	no_status	❓
install-pkg (windows-2022, lightning, 3.11)	no_status	❓
install-pkg (windows-2022, notset, 3.8)	no_status	❓
install-pkg (windows-2022, notset, 3.11)	no_status	❓

These checks are required after the changes to .github/workflows/ci-pkg-install.yml, src/lightning/__setup__.py, src/version.info.

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