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

[tmat]

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.

[marcpopMSFT]

@aortiz-msft nuget part of the dotnet CLI code so doesn't make sense to add the label and move to nuget repo but someone from your side should take a look.