Compute wheel content hashes once

[ralbertazzi]

installer computes hashes over a wheel both on:

• Validation, provided validate_contents = True
• Installation

This isn't ideal when validating and installing gigantic wheels, such as this 2.1 GB torch distribution.

• Installation with validation with validate_contents = True takes 97 s
• Installation with validation with validate_contents = False takes 55 s
• In comparison, pip install (used through poetry with config installer.modern-installation false) takes 51 s

It would be great to keep content validation while avoiding computing hashes twice. I tried to implement it in a hackish way in python-poetry/poetry#8027, but this should be solved inside installer itself.