Description of the problem, including code/CLI snippet

Listing package files appears to be unsupported. The API endpoint was introduced in GitLab 11.8.

Expected Behavior

Listing package files should be possible.

Actual Behavior

Listing package files is not possible.

Specifications

• python-gitlab version: 2.6.0
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): gitlab.com

PR incoming.