This adds a test to our buildkite suite called 'manifest' that installs check-manifest and runs it against every library package.
This diff removes some empty requirements.txt files, but otherwise does not change the package contents in sdist.
Instead it configures setup.cfg to instruct check-manifest to ignore certain files that are not included. We should potentially reconsider and start including test files and examples in the manifest, but that should probably be a separate diff.
This would have caught the manifest bug fixed by 4d1cec096.