warn when overwriting file at installation

[dimbleby]

see #8382 for context - if two packages install the same file it would be useful to know this.

I'm in two minds about this warning, I expect both true and false positives are rather rare and it's hard to evaluate which matters more

• this warning would surely have given a clue in 0-byte files getting unpacked #8382: knowing about it when it really is unwanted seems very desirable

  • but I suppose such things hardly ever happen

• adding this warning will likely expose this repository to a small trickle of issue reports, of varying legitimacy

  • in most cases the answer will be: yes, that's a badly built wheel. cf Ignore existing script files pypa/installer#170, 0.7.0: issue with installing pecan .whl archive pypa/installer#182, Unable to install salt wheel pypa/installer#194
  • there may be a tiny number of non-native style namespace packages out there for which this is expected behaviour, in which case the answer will be "yes, please just ignore the warning".
    • Though in that case I expect things get broken when uninstalling, certainly nothing is tracking "this file belongs to two packages, keep it until they both are uninstalled"

IMO probably a net gain, but I don't care enough to fight for it.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.