Warn if `doc/api/index.html` exists when publishing (an artifact of dartdoc)

[jonasfj]

If there is an doc/api/index.html we should offer a suggestion that dartdoc results should not be included in the uploaded package.

Credits @isoos in https://github.com/dart-lang/pana/issues/521 for suggesting this.

[sigurdm]

Still relevant. I think we should add it to the implicit gitignores here: https://github.com/dart-lang/pub/blob/3894534bdcbdf0308486152bacf19cd69110c559/lib/src/package.dart#L232

@jonasfj thinks it is too risky in case somebody wrote some actual (non-dartdoc) documentation in doc/api, and a warning is better.

Until now there is no way to explicitly ignore a warning.

Usually people will see these files in git status and add a gitignore anyway.