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

This issue has been migrated from [#14788](https://github.com/matrix-org/synapse/issues/14788).

---

Repro steps:

* Run `poetry install --extras all -v` on a git checkout
* Interrupt it due to https://github.com/python-poetry/poetry-core/pull/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.