Add `nuget why` and `nuget package` command definitions

These commands are currently defined in `NuGet.CommandLine.XPlat.Commands` and only added to the implementation.
Parser that only uses command definitions is thus not able to parse them.

To allow adding nuget commands without cross-repository changes the NuGet repo can define "NuGet.CommandLine.CommandDefinitions" source package that gets included in `Microsoft.DotNet.Cli.Definitions`.

Tracks TODOs in code.