deprecate `pub` command

[jonasfj]

Things to do:

• Migrate flutter pub to not wrap pub, but dart pub instead.
• Migrate travis-ci scripts to use dart pub get (for dart >= 2.10.0) (wontfix for now)
• Dart-Code#2944 Migrate vscode integration to use dart pub get (for dart >= 2.10.0)
• WEB-48176: Migrate intelij integration to use dart pub get (for dart >= 2.10.0),
• Migrate pana to invoke through dart pub (Migrated from "pub" -> "dart pub" pana#852)
• Migrate pub-dev to use dart pub (Migrated from "pub" -> "dart pub" pub-dev#4655)
• Add deprecation warning when pub is invoked in a terminal.
• Change pub to be a wrapper around dart pub.

[sigurdm]

Migrate flutter pub to not wrap pub, but dart pub instead.

We probably want to wrap dart --disable-dartdev-analytics pub instead, so we don't send analytics twice.

[sigurdm]

Migrate travis-ci scripts to use dart pub get (for dart >= 2.10.0)

Do we still need this after migrating to github code actions?

[jonasfj]

note to self, patching travis involves:

• https://github.com/travis-ci/travis-build/blob/master/lib/travis/build/script/dart.rb
• https://github.com/travis-ci/travis-build (which can create a shell script from a travis.yaml file)

[mit-mit]

@sigurdm can this be closed now?

[sigurdm]

I managed to commit out of order - we can close whenever: flutter/flutter#88509 lands. I hope to land it before flutter discovers that the sdk is broken for them :|