This switches to [Go modules](https://go.dev/blog/using-go-modules), which have now been the standard for dependency management in Go codebases for a while. In addition to initializing a new Go module, it also updates the paths of some gopkg.in dependencies, which are deprecated as well.
Change-Id: Ie5c9faa415a65ab76cbe59f4afb437a9250be392
Signed-off-by: Moritz Sanft <58110325+msanft@users.noreply.github.com>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11773
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
