Do not overwrite files from already-installed packages

[davidedelvento]

Following the suggestion in pypa/pip#988 (comment) I am submitting this test case here.

Instead of just repeating myself, the issue is pypa/pip#4625

Not sure if that is in scope with the new dependency resolver, but mentioning it and let you think about it for a moment can't hurt.

[davidedelvento]

This and #17 are two different things. This one is the most serious and is about pip silently overwriting files from previous packages it installed. In brief, if two unrelated packages a and b both have a foo.py file, in some circumstances (see pypa/pip#4625 for details) the second that gets installed overwrites the first, and pip does not notice.

Again, I am not sure if solving the issue is in the scope of what you are working on at the moment, but...

[pradyunsg]

Okay, I closed this for the wrong reasons earlier. However, this is not in scope for this project and is the same request as pypa/pip#4625.

I'm going to go ahead and close this in favor of that issue.