Improve errors/warnings for unnecessary .ko.yaml settings

[imjasonh]

From #530 (comment)

A few points to note:

• The -B (--base-import-paths) and --bare flags are not designed to be used together, and in this case, -B takes precedence: https://github.com/google/ko#naming-images

• The main field in .ko.yaml is a directory, not a full Go import path.

• If your .ko.yaml and go.mod are in the same directory as your main package, and if this directory is also your working directory, then you can leave out dir and main.

• ko sets CGO_ENABLED=0 by default.

Each of these could be warnings or errors to help make .ko.yaml easier to use and less needlessly verbose.

[imjasonh]

Another one:

• ko publish --local is confusing if KO_DOCKER_REPO is set to anything besides ko.local, and even then it'd be nice not to have both options. Maybe we should just deprecate --local, but even if we don't we should at least log a warning about it.

[imjasonh]

For all of these I think we should log a warning, and note that they might become failures in a future release.

Then, after we cut a release (v0.10), we can make them failures.

[imjasonh]

Another:

• ko build ./ --platform=all,linux/arm64 only builds for linux/arm64 and doesn't even log a warning. This should probably just fail if you have all and anything else.