refactor: simplify Wheel API

[baszalmstra]

This PR is in preparation of introducing cached wheels that are not zips. The Wheel struct has a lot of responsibilities (installing, metadata, lazy metadata, vitals), I wanted to simplify it so that it represents just a zipped .whl file.

I changed the API by moving a lot of the functionality associated with wheels out of Wheel and into their own functions.

• Unpacking (now called install_wheel) is no longer an associated function. I did this because in the future Ill change the API to also be able to install cached wheels. I also moved this to its own module. It seemed appropriate.
• get_vitals(). This function was extracting data from a .whl that was used in different places. The metadata was used by the packagedatabase and some other fields where used during installing. Neither usecases required all data so I split it up and moved the extraction to where it was actually used. get_metadata is still part of Wheel.
• lazy_read_metadata. This function had nothing to do with Wheel. I moved it to its own function.
• I removed all code that become unused because of this.

[tdejager]

Looks a little better! I'm a fan of keeping the struct methods to a minimum so this is great :)