* enforce the use of goimports
Instead of enforcing `gofmt`, enforce `goimports`, which also asserts
a separate section for non-builtin packages.
* run `goimports` everywhere
* exclude generated .pb.go files
This PR does two things:
- Only run the tests when relevant files change.
- Cache the Go Modules directory between runs, so they don't have to redownload everything everytime (go.sum is the cache key). Pretty much straight from the examples: https://github.com/actions/cache/blob/master/examples.md#go---modules
Wade Simmons