"Add Dependency" doesn't handle flutter SDK checks properly for multiple packages #6028

We now support multiple packages in a single "Add Dependency" call. You can write "foo bar" and install both packages together.

However the check for whether to use --sdk flutter is based on the input being a single package. It's not clear if pub add supports a mix of SDK/non-SDK packages, so we might need to make some changes here to use multiple commands.