I am using v3.27.2 of the extension and I searched for a similar issue but I didn't find anything related.

Currently, the auto-completion about dependencies just works in pubspec.yaml but I think it can be better to add the dependencies in import (for tired guys can be the best solution).

At this time the process for using a package is: (expect its kind - local, hosted on pub server or as a dev dep)

1. Adding dependencies in pubspec.yaml
2. Runnin pub get to fetch the package and other related configs
3. import it and use it

But I suggest a new way (I am not sure this feature can be implemented in extension or not let me know).
For example, we want to add a dependency in the dependency section in pubspec.yaml what we should do is:

1. import the package with this syntax import 'dep:<package_name | git_remote_repo | local/path/to/package>'; auto completion works as same logic as used in pubspec.yaml (with some modification)
2. crtl + . click on fetch package
3. then pub add <package_name> --<associated_switch> will run and do all configurations

Note: as dev dependency we can use import 'dev:<package_name | git_remote_repo | local/path/to/package>';