Consider moving ./cmd/apko/main.go to ./main.go

[imjasonh]

ko used to be in ./cmd/ko, but @n3wscott correctly pointed out that makes the go install instructions needlessly long and stutttery, so we moved it to ./main.go and made ./cmd/ko/main.go log a warning then call the usual cobra root entrypoint.

See ko-build/ko#257 for the whole saga.

If you think apko will follow a similar path, it might be worth moving it up now, while there are few enough users.

If this sgty I can send a PR.

[kaniini]

Go for it!