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 dart-lang/pana#521 for suggesting this.

[sigurdm]

Still relevant. I think we should add it to the implicit gitignores here:

pub/lib/src/package.dart

Line 232 in 3894534

static final _basicIgnoreRules = [

@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.