Invalid command '.' with virtualenv.install_package(installer='pip')

The readme indicates to use `virtualenv.install_package('coverage', installer='pip')`, but if I use that command as `virtualenv.install_package('.', installer='pip')`, it fails with `Invalid command '.'` and the command it's trying to run is `pip -q .` (no `install` command). That's unexpected. I would have expected `installer='pip'` to invoke `pip install`.

Start with the README example for virtualenv.install_package and trace how installer='pip' constructs the command. Reproduce the failure with virtualenv.install_package('.', installer='pip') and compare it with the documented coverage example. Done means the pip installer invokes the expected install command and the relevant behavior is covered by a test.

Written by the indexing model from the issue text.