Change go.mod and import paths to reflect project path

This is a fairly simple MR which updates the go.mod file and the code for this utility to reflect this project's path. Prior to this change, it was not possible to install this tool locally with go install -v gitlab.com/gitlab-com/content-sites/handbook-tools/hugolint@latest as the paths pointed to a different project, preventing a working build outside of a local check out of this project.