Move command completion configuration from definitions to implementations. Completion is not needed for command line parsing.
The completion often depends on the implementation of the command (e.g. NuGet package downloader, etc.).

The last commit switches the affected commands to a new pattern: instead of definitions being static classes and arguments, options and subcommands stored in static fields we instantiate command definitions.
This allows us to use inheritance for similar command definitions (e.g. package add and add package) and access their options/arguments in a strongly typed manner.

Will move all commands to the new pattern in follow up PRs.