Allow a multiple-file selection for pub operations

The Project menu allows running 'flutter packages get' from the menu when a single pubspec.yaml file is selected. If more than one is selected it should iterate over the list, running it for each.

See https://github.com/brianegan/flutter_architecture_samples for an example of where this would be useful.

Start at the IntelliJ Project menu action that runs “flutter packages get” for a selected pubspec.yaml, and trace how the selected files are handled. Extend the behavior so multiple selected pubspec.yaml files are processed, then verify that the command runs once for each file.

Written by the indexing model from the issue text.