Auto merge of #13845 - birryree:package-under-target-profile-dirs, r=aneeshusa

Part of #13551 - Create packages under release/debug directories as appropriate

r? @aneeshusa

This change implements fixes for #13551 for Linux and MacOS targets.

`/python/servo/package_commands.py` was modified so that:
- On MacOS, it creates all intermediate packaging directories like `dmg`, `brew`, and `brew-tmp` under `target/(release|debug)`, rather than in `target` directly.
- On MacOS, all packaging artifacts (`.dmg`, brew `.tar.gz`) are packaged under `target/(release|debug)`, rather than in `target` directly.
- On Linux, the resulting `tar.gz` Servo package is placed under `target/(release|debug)`, rather than in `target`.
- Also did some extra cleanup around path parsing in the MacOS packaging code, to use `os.path` methods rather than straight `'/'` parsing with `split` and `join` where it was applicable.

`/etc/ci/upload_nightly.sh` was modified to:
- Look for artifacts in `target/release` for `mac`, `macbrew`, and `linux` platforms, rather than just `target/`.

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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #13551
- [x] These changes do not require tests because it is based on CI and packaging tools. They were manually tested for correctness.

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…lease/debug as specified or detected. Modify macos packaging to create all packages under release/debug directory. Updated etc/ci/upload_nightly.sh to support uploading from either release/debug directory, depending on what was built

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