Use git submodule for the Custodian Go module

This PR brings a Git submodule for the Custodian Go module.

Prior to that, every component was referencing to the Go module using its Gitlab URL, which lead to unnecessary overheads during development. With the Git submodule, the Custodian Go module is only referenced locally, which makes the development easier.

The Git submodule is referred to as pkg in this repo. The Dockerfiles have been updated to add it to the images.

All go.mod files have been updated with the following line

replace gitlab.com/data-custodian/custodian-go => ../../pkg