feat: Add PluginMetapackage.Sdk

[amis92]

As demonstrated in samples/MetapackageSample, this Sdk enables easy creation of plugin metapackage for generator creators:

• allows inserting arbitrary <dependency> elements into nuspec before actually packing it
• implicitly references CodeGeneration.Roslyn.Tool

It's a game changer for generator creators, because without touching any nuspec or using complicated .targets, or having multi-step builds, they can easily produce NuGet packages that in turn make their consumer's experience nice and simple.

[garyng]

Is this on nuget already? The nuget link on readme is 404 though..

[amis92]

Is this on nuget already? The nuget link on readme is 404 though..

It is not yet published. I'm still working on it. Please follow ("watch") releases on this repo to keep track :)