Refactor, reorganize packages

What does this MR do?

Reorganize the packages in preparation for gitlab-org/gitlab#14630 (closed).

• extract the client package, to be removed
• extract the advisory package, to be totally revisited
• introduce a scanner.Scanner, to make the main func easier to read
• extract scanner/finder

It would make sense to move parser to the top-level package but it would break backward compatible and create conflicts in open MRS.

What are the relevant issue numbers?

gitlab-org/gitlab#14630 (closed)