Sometimes it is useful to fork a dependency in a different repo and
then use that forked code in Kubernetes. Normally one would do `go mod
edit -replace ...=/local/path` but that has drawbacks:
- repos under staging are not updated
- sharing the modified Kubernetes with others is harder, for example
in a WIP or RFC PR
The revised pin-dependencies.sh supports this with an optional
=<replacement> part in the dependency parameter.
Determining the revision upfront with `go mod download` also makes the
script simpler.
