Suggestions don't work when tool is invoked as a dotnet subcommand

I have a tool named `dotnet-shift`.

The tool can either be invoked as `dotnet-shift`, or it can be invoked as `dotnet shift`.

When the former syntax is used to invoke the tool, the command completions work.
And when the latter is used, they don't.

My understanding is that for this to work, the .NET SDK should pass on the request for suggestions to the tool that gets invoked.

cc @jonsequitur @adamsitnik