Warn that poetry will self-sabotage with zero-length setup.py files if interrupted.

[matrixbot]

This issue has been migrated from #14788.

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Repro steps:

• Run poetry install --extras all -v on a git checkout
• Interrupt it due to Speed up builds with large number of git ignored files python-poetry/poetry-core#499 not having been merged, and the fact that it gets stuck traversing a massive media store directory tree.
• Don't notice that it's left a zero-length setup.py file in the root of the checkout
• Run poetry install --extras all -v again
• Discover it fails cryptically with:

  Obtaining file:///home/matrix/synapse
    Preparing metadata (setup.py): started
    Preparing metadata (setup.py): finished with status 'done'
  ERROR: No .egg-info directory found in /tmp/pip-pip-egg-info-81uum26d

• Eventually figure out that you have to delete setup.py to make poetry stop mysteriously failing.

Filing this in case we want to update the Synapse guide to reflect this - or failing that, to leave a breadcrumb trail for others to find. Or i guess we could ask poetry to fix this horrific behaviour; i had to strace the wretched thing to figure out what the root cause was.