October 2021, Roger H. Jörg
The two packages NuGet.Packaging and NuGet.Protocol seem to fulfill the needs for my tasks and commands.
The current (as of this writing) NuGet.config file contains the following section:
<packageSources>
<clear />
<add key="Nuget" value="https://api.nuget.org/v3/index.json" validated="True" trusted="True" />
<add key="Packages" value="file:///C:/source/tldag-dotnet/packages" />
</packageSources>
The following code snippet successfully found the TLDAG.Sdk package that is created during the build of the project.
ILogger logger = NullLogger.Instance;
CancellationToken cancel = CancellationToken.None;
SourceCacheContext cache = new SourceCacheContext();
string root = Environment.CurrentDirectory;
ISettings settings = Settings.LoadDefaultSettings(root);
PackageSourceProvider provider = new(settings);
PackageSource packageSource = provider.GetPackageSourceByName("Packages");
SourceRepository repository = Repository.Factory.GetCoreV3(packageSource.Source);
FindPackageByIdResource resource = repository.GetResource<FindPackageByIdResource>();
IEnumerable<NuGetVersion> versions = resource.GetAllVersionsAsync("TLDAG.Sdk", cache, logger, cancel).Result;
foreach (NuGetVersion version in versions) Debug.WriteLine(version);