refactor(#9): move gitlab logic to specific repository package

[sacha-c]

A first step to adding github is to isolate gitlab. This PR replaces all uses of https://pkg.go.dev/github.com/xanzy/go-gitlab#section-readme with generic types in our repo package, so they can be used regardless of the type of repository host.

• Renames gitlab package to repo
• Moves common "repo" types to a repo.go file in which we have the common service interface & structs (Project, Issue)
• Refactors gitlab.go to implement the common interface
• Refactors the rest of the CLI to use the common interface rather than the specific gitlab structs

With this change to add github I expect we will need to:

1. Create a new repo/github.go service which implements the common interface
2. Add logic in patrol to provide a different repository service based on the path type (gitlab vs github)