Go: Completely rewrite

[micahflee]

The existing Flatpak Go Get Generator no longer works with the modern Go ecosystem. The flatpak-go-get-generator.py script works by looking for git repos in the Flatpak build dir after running go get, but that doesn't even create git repos anymore.

So this PR deletes the old go-get folder and makes a new go folder. This requires that Go and git are installed on the local system.

This script works by creating a new Go module in a temporary folder, add the given Go package as a dependency, and then runs go list -m all to get the full list of Go modules. For each module, it uses go list -m -json <module> to get detailed information. And then finally, it outputs the module in YAML format.