remove the `package` param from pip install function

Remove package param as it's useless.

Also, use the `$name` to replace `$package` adds one more benefit, now I can do something like this:

```puppet
$pip_package = [
  'celery',
  'gevent',
]
python::pip::install { $pip_package:
  venv   => $venv,
  ensure => present,
}