Fixes: NuGet/Home#4126Fixes: NuGet/Home#8668
Spec link: https://github.com/NuGet/Home/blob/dev/designs/DotNet-Sources-Support.md
## Main Work - `dotnet nuget add|remove|* source` implementation
Moves and refactors existing `NuGet.exe sources add|remove|*` command into NuGet.Commands.csproj, so that it can be called by NuGet.exe and by DotNet.exe.
## Localization fixes were then necessary
Dealt with some localization challenges as part of the strings that moved into NuGet.Commands.dll. Needed to make the ILMerge command that makes NuGet.exe to get localization strings from the 13 NuGet.Commands.resources.dll.
## CodeGeneration templates and datafile for commands/verbs
Also implemented a CodeGeneration engine (see .tt files) based on data in commands.xml - which automated code creation of much of the infrastructure. As we add more commands into dotnet.exe, this should be useful.