Introduce .pubignore

[sigurdm]

Implements .gitignore style files called .pubignore.

Files ignored by a .pubignore will not be published (no matter if they are checked in to git).

A .pubignore in a folder will override a .gitignore if both are present in the same folder.

pub will now no longer call out to git to have .gitignore files interpreted.
This results in some subtle changes in the package listings:

• .gitignore files are taken into account even if the root is not a git directory.
• global gitignore files are not taken into account.
• files that are ignored by a .gitignore but checked into git, would be listed by pub previously, but now they are ignored by pub.
  • Added a publish-time validator to warn against this case (it is most likely a mistake when this happens, and can always be avoided by writing more .gitignore rules).

Changed package.listFiles to always take ignore files into account - no usecases warranted the difference.

Most implementation of ignore.dart by jonasfj@

fixes: #2222
fixes: #2786
fixes: #2566

[jonasfj]

We had some discussions, consider the following options:
(A) .pubignore in any folder (same syntax as .gitignore).
(B) files: ['glob/pattern/for/**/file.dart', ...] in pubspec.yaml (if present, only files matching a pattern is included)
(C) include: ['glob/pattern/for/**/file.dart', ...], exclude: ['glob/pattern/for/**/file.dart', ...] in pubspec.yaml
(D) exclude: ['glob/pattern/for/**/file.dart', ...] in pubspec.yaml

I'm personally attracted to (A) because:

• It's the same syntax as .gitignore,
• Any glob-patterns we invent won't be as expressive as .gitignore (e.g. / has special meaning in the start of a pattern),
• The .pubignore file itself should not be included when publishing the package:
  • If user is ignoring private files they don't want to share, they might not want to share the file-patterns they are ignoring,
  • This is information that isn't needed in the package,
  • Options (B), (C), (D) needlessly increases the pubspec, which increases the size of all responses from the list-versions API.
• Like a .gitignore file, you can have a multiple .pubignore files, you don't need to put in the root directory. If you are generating some temporary files, you can simply generate a .pubignore file in said directory too to avoid publishing any of them.

@sigurdm, @isoos thoughts?

[sigurdm]

Yeah - the main complaint about .pubignore files is that we now have even more ("hidden") files floating around.

But for most users pubignore will not be needed.

[jonasfj]

Let's pull this into dart-sdk ASAP, so we can get some testing in dev releases..