Warn in pub publish if git status is not clean

[mklim]

Splitting this off from some discussion on flutter/flutter#27258. When we're publishing a package, it's possible to have build artifacts or other untracked/ignored files in the local directory of the package. But by default we only ever want to publish files that are already tracked and committed in git. pub publish does spit out a tree of all the files its planning on publishing already, but in practice this isn't really useful for us since there's so many files in a typical package.

It would be helpful for us if there was a flag or setting where we could tell pub that we're trying to use git as our source of truth, and have it at a bare minimum panic and throw up a warning if the git status isn't clean and/or it's about to upload files that aren't already committed in the source tree.

[jonasfj]

Maybe we should just warn against publishing files that are:

• tracked in git
• have changes not committed in git

I think it's always reasonable to warn against tracked files with changes not committed in git.

[jonasfj]

I think this can be implemented as a validator:
https://github.com/dart-lang/pub/tree/master/lib/src/validator