fix: move third_party.bigframes_vendored to bigframes_vendored

[tswast]

This will avoid potential conflicts when run from a location that contains the commonly named third_party directory.

Note: I followed the instructions here: https://stackoverflow.com/a/17179022/101923 but I also had to add a pyproject.toml file to support editable installations (see: pypa/setuptools#230 (comment)).

Fixes internal issue 328781348.
🦕

[GarrettWu]

Just curious. If we want to avoid third_party, why can't we just remove it from the path?

[tswast]

Just curious. If we want to avoid third_party, why can't we just remove it from the path?

We need it to signify that this code has a different copyright (isn't covered under Google CLA) per go/thirdparty/non-google3 policy.

[tswast]

Note: please don't manually merge unless this has been synced to main. I'm using automerge: exact which should ensure this change only merges if synced to main when merged via automerge.

Edit: Update: automerge: exact doesn't affect if it's synced with main or not. It affects if the review is on the latest commit inside this pull request, which isn't really what I wanted. I've instead updated the Pull Request settings to always suggest update to main but it's not a requirement still.

This review does not reference the most recent commit, and you are using the secure version of merge-on-green. Please re-review the most recent commit.