Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/CONTRIBUTING.md

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+# Contributing to Buck2 Prelude
+
+This repository is a subset of <https://github.com/facebookincubator/buck2>.
+You can contribute to either that repo, or this repo - changes will be mirrored to both.
+
+We want to make contributing to this project as easy and transparent as possible.
+
+## Our Development Process
+
+Buck2 Prelude is currently developed in Facebook's internal repositories and then exported
+out to GitHub by a Facebook team member; however, we invite you to submit pull
+requests as described below.
+
+## Pull Requests
+
+We actively welcome your pull requests.
+
+1. Fork the repo and create your branch from `main`.
+2. If you've added code that should be tested, add tests.
+3. If you've changed APIs, update the documentation.
+4. Ensure the test suite passes.
+5. Make sure your code lints.
+6. If you haven't already, complete the Contributor License Agreement ("CLA").
+
+## Contributor License Agreement ("CLA")
+
+In order to accept your pull request, we need you to submit a CLA. You only need
+to do this once to work on any of Facebook's open source projects.
+
+Complete your CLA here: <https://code.facebook.com/cla>
+
+## Issues
+
+We use GitHub issues to track public bugs. Please ensure your description is
+clear and has sufficient instructions to be able to reproduce the issue.
+
+Facebook has a [bounty program](https://www.facebook.com/whitehat/) for the safe
+disclosure of security bugs. In those cases, please go through the process
+outlined on that page and do not file a public issue.
+
+## Coding Style
+
+We use several Python formatters.
+
+## License
+
+By contributing to Buck2 Prelude, you agree that your contributions will be
+licensed under both the [LICENSE-MIT](LICENSE-MIT) and [LICENSE-APACHE](LICENSE-APACHE)
+files in the root directory of this source tree.