Enable packing legacy PackageReference projects without the need for a package (ie merge NuGet.Build.Tasks.Pack into NuGet.Build.Tasks)

[nkolev92]

The PackageReference functionality has 2 targets, restore and pack, which are deliver by 2 separate packages/targets files created by NuGet.Build.Tasks and NuGet.Build.Tasks.Pack and

We have 2 types of PackageReference functionality, SDK projects and classic csproj projects.

For SDK projects, they're both available by default, although they're imported from 2 different places.
For legacy projects, the restore target is available , but the pack target is not. It requires you to add a PR to NuGet.Build.Tasks.Pack.

Another feature of NuGet.Build.Tasks.Pack is that it allows you to overwrite the default pack targets, allowing for faster onboarding for pack fixes.

Beyond the UX, the issue with pack and restore being separate is that pack is ilmerged, so we're effectively shipping duplicates of most assemblies.
We should decide whether we want to preserve the bootstrapping ability of NuGet.Build.Tasks(.Pack) or if it's sufficient to just make it available in all relevant scenarios.
Final consideration is that making the pack targets available for all legacy projects could cause a different behavior when people run the "pack" target, so we'd need to consider whether we condition the import for legacy projects on something.

[nkolev92]

cc @zivkan @jeffkl

If we do this, we don't need: #13079.

[nkolev92]

Either way, we should probably stop publishing https://www.nuget.org/packages/NuGet.Build.Tasks#versions-body-tab, like https://github.com/NuGet/client.engineering/issues/2582 suggests.