add lint/check to avoid missing entries in requirements.txt/pyproject.toml

[mr-tz]

as encountered in #2359

[williballenthin]

in CI we install deps from requirements.txt and then install from pyproject. but the second step should be a no-op, except to install capa. so maybe we can inspect the pip output to assert there are no extra packages being installed. or we could do pip freeze before/after and ensure the diff is a single line.

these aren't lints that we can run standalone but would help us catch things along the way