[Pack] Support for Analyzers, MSBuildSdks, Templates

[amis92]

Details about Problem

There is a lot of .NET Core related package types that currently have no good development story:

• Templates (some problem with templates: #6862)
• MSBuildSdks
• Analyzers (also #6793)
• possibly other types that don't really have TargetFramework, or a tooling package with bundled dependency dlls like DotnetTool does: https://github.com/NuGet/Home/issues/3891

Possibly associated issue: https://github.com/NuGet/Home/issues/4254 (allow dotnet pack to pack nuspec file without needing a csproj file)

All of them require usage of .nuspec file, which is very inconvenient in the new world of .NET, mostly due to having to pass all the variables already generated/provided in msbuild system, like PackageId, Author, Version (essentially all the metadata) as NuSpecProperties.

Also this requires actually building and maintaining the nuspec file, which has unique syntax, and, most importantly, doesn't seem to be promoted as the way forward.

I ask for the appropriate Targets, Sdks, features and project types (maybe .packproj or sth), so that I can build the packages listed as easily as the DotnetTool packages, with MSBuild tooling/dotnet CLI with no requirement to use nuspec and/or manually map and duplicate metadata properties.

NuGet product used: dotnet.exe

dotnet.exe --version: all up to 3.0.100-preview

[nkolev92]

Analyzers ask captured in https://github.com/NuGet/Home/issues/6793