Before this commit, when single-file app was detected, the behaviour was to fallback on DLL scanning. But DLL scanning would not find anything for an app published as a single-file, by sheer definition of single-file app!

After this commit, an exception is thrown if the app is published as a single-file AND no `Serilog:Using` section is defined in the configuration. The error message explains that either a `Serilog:Using` section must be added or show how to explicitly configure assemblies through the `ConfigurationReaderOptions`.

This is when the default DependencyContext is actually null.

Also run all the tests on .NET 6 which is a Long Term Support (LTS) release.

Use a HashSet instead of a Dictionary keyed by the assembly full name. If an assembly is loaded twice with the same AssemblyName it will be the same instance so a HashSet (without even a custom IEqualityComparer<Assembly>) is the perfect solution.

Also, Assembly.Load can throw many exceptions but won't return null.

…works

… command line

This works around dotnet/sdk#27202
It also has the benefit of using settings _only_ from the specified config file, ignoring the global nuget.config where package source mapping could interfere with the local source.

Add a new PublishAsync(PublishMode) method and also add a comment explaining why it would not work to publish multiple apps in parallel.