Move Go code up one level

This MR moves all the code in go/ up by one level, so we have top-level cmd/ and internal/ directories.

This requires the go.mod and import path to change for all the Go files, so this is done.

It also breaks the build, so we replace the existing (ruby) compile and format scripts with Makefile stanzas. This is a big step towards completing #213 (closed), and easier to understand than modifying the Ruby, I feel.

Once again, this is a big-diff, small-cognitive-load MR. Probably best appreciated via commit-by-commit review.

Part of #173 (closed)