Convert to CodeQL workspace and CodeQL packs #26

Background

Currently this repository includes the github/codeql repository as Git submodule, and then uses its CodeQL libraries. Back then this was the standard way to set up such a project (?), but in the meantime CodeQL workspaces and CodeQL packs were added.

Feature description

Convert this repository to use CodeQL workspaces and CodeQL packs instead of a Git submodule.

Have to research which approaches exist for CI respectively automatic updating of the CodeQL dependency and CodeQL CLI version.