This change makes `flutter pub` show the help usage:
```
PS > flutter pub
Commands for managing Flutter packages.
Global options:
-h, --help Print this usage information.
-v, --verbose Noisy logging, including all shell commands executed.
If used with "--help", shows hidden options. If used with "flutter
doctor", shows additional diagnostic information. (Use "-vv" to force
verbose logging in those cases.)
-d, --device-id Target device id or name (prefixes allowed).
--version Reports the version of this tool.
--suppress-analytics Suppress analytics reporting for the current CLI invocation.
--disable-telemetry Disable telemetry reporting when this command runs.
Usage: flutter pub <subcommand> [arguments]
-h, --help Print this usage information.
Available subcommands:
add Add a dependency to pubspec.yaml.
cache Work with the Pub system cache.
deps Print package dependencies.
downgrade Downgrade packages in a Flutter project.
get Get the current package's dependencies.
global Work with Pub global packages.
login Log into pub.dev.
logout Log out of pub.dev.
outdated Analyze dependencies to find which ones can be upgraded.
pub Pass the remaining arguments to Dart's "pub" tool.
publish Publish the current package to pub.dartlang.org.
remove Removes a dependency from the current package.
run Run an executable from a package.
test Run the "test" package.
token Manage authentication tokens for hosted pub repositories.
upgrade Upgrade the current package's dependencies to latest versions.
uploader Manage uploaders for a package on pub.dev.
version Print Pub version.
Run "flutter help" to see global options.
```
Previously it showed an error message:
```
PS > flutter pub
Missing subcommand for "flutter pub".
Run 'flutter -h' (or 'flutter <command> -h') for available flutter commands and options.
```
Addresses https://github.com/flutter/flutter/issues/110025
