List packages under a given path with `pip list`

[lkollar]

What's the problem this feature will solve?

This is the same feature proposed in #6404 for pip freeze. Given that the two commands perform similar functions, they should both be able to list packages installed with pip install --target. Support for --path in pip freeze is being added in #6450. @xavfernandez suggested implementing this in pip list.

Describe the solution you'd like

A new --path argument for pip list which lists installed packages under the given path.

As the proposed functionality is identical to what #6404 describes, please see that issue for further information on the below points.

Alternative Solutions

Additional context

[lkollar]

I'm happy to work on this once #6450 lands.