refactor: share the case-insensitive lookup of data files

The scenery repository's include lookup becomes CaseInsensitivePaths, which
takes any folder over IFileSystem and gives a name as it is on disk. Of
folders named alike in other case it now tries each, not only the first.
A name is joined to the folder as the simulator joins it, a leading slash
counting for nothing. IFileSystem tells whether the disk ignores case; there,
as on Windows, a name missed as written is not looked for any further, so
no folder is listed.

StarterNG.Tests/Fakes/InMemoryFileSystem.cs

@@ -97,6 +97,8 @@ public sealed class InMemoryFileSystem : IFileSystem
         return this;
     }
 
+    public bool IgnoresCase => _comparison == StringComparison.OrdinalIgnoreCase;
+
     public bool FileExists(string path) => _files.ContainsKey(Normalize(path));
 
     /// <summary>A link counts as the directory it points at, as the disk has it.</summary>