Warn in "pub publish" if bin/ is non-empty, but bin/<packageName>.dart does not exist

[jonasfj]

The default entry point for a pub package is bin/<packageName>.dart, so if publishing a package that contains bin/ (non-empty), we should give a warning if bin/<packageName>.dart doesn't exist.

[jonasfj]

Maybe a warning is too strong... there are legitimate reasons to not include a default binary.

So maybe it should just be a hint.

[jonasfj]

We could limit to only warn if bin/*.dart exists and there is no bin/<packageName>.dart.

[sigurdm]

Do we still want this?