Support NuGet packages defined in .csproj #3

The NuGet workflow action scripts currently assume that the package to create and upload are defined in a .nuspec file. However, NuGet packages built in other .NET versions may instead have the package defined in their .csproj file.

This should be done with additional logic in the existing NuGet action scripts to identify which type of package build to do. If that is not going to be feasible, it could be a set of separate internal + public release scripts for .csproj packages specifically.