Review and cleanup go.mod

[ethanfrey]

Is your feature request related to a problem? Please describe.
A look at go.mod diffs after a recent PR #663 showed a number of changes in "indirect" packages. Also many dependencies that were unexpected.

Describe the solution you'd like
Let's go through and make sure nothing unnecessary is in this. We also should reflect on how we handle versioning for tools (like prototool). There are interesting related discussions in golang/go#25922 and golang/go#27653

In particular golang/go#25922 (comment) seems like a proper solution here and may allow us to reduce the surface of go.mod.

Additional context
There is likely some overlap with other protobuf tooling cleanup in #629