Avoid breaking developer builds during gen-code integration

[jcburley]

Various changes made in the gen-code branch not only produce additional a_*code.go files, but change the way a_*data.go files are generated.

So it's best to start with a clean slate before running go generate ./..., to prevent it picking up those stale files.

Another approach to this could be to use build tags so go generate-run tools don't pick up any a_*.go files at all, but this seems easier and more effective.

[candid82]

Thanks!