Consider warning users about shell path hashing on upgrade

[chrahunt]

What's the problem this feature will solve?

Shells can remember paths to commands, so subsequent invocations do not have to search PATH. This causes confusing behavior when executing pip install --upgrade pip, but a regular pip invocation still resolves to the system pip (at /usr/bin/pip) and import fails.

Describe the solution you'd like

On package upgrade, check whether the parent process is bash (or any shell that does this). If so, check whether the script that was just installed also exists anywhere else in PATH. If so, print a warning to the user that they may need to execute e.g. hash <script1> <script2> ... before the scripts will work correctly.

Example where this would've helped: #6332.

Alternative Solutions

1. Ongoing user support
2. Unconditionally ask users to close and reopen their shell

Additional context

• Upgrade fails #6332
• ImportError in system pip wrappers after an upgrade #5599

[uranusjr]

+1 on this; I see more and more tools providing similar help messages nowadays. It is a good approach IMO. (Although we might want to perform some basic platform/shell detection for this; suggesting hash in e.g. PowerShell would be very confusing to users.)

[pradyunsg]

I'm on board.