C# autobuilder: Build all solutions at the highest depth

The most common reason for the C# autobuilder to fail is because it
cannot determine a single unique .sln or .proj file to build, instead
reporting multiple sln or proj files at the same shortest depth. This
commit changes this to build all such files, rather than reporting an
error.

Author: lcartey

csharp/autobuilder/Semmle.Autobuild/Autobuilder.cs

@@ -156,15 +156,9 @@ IEnumerable<IProjectOrSolution> FindFiles(string extension, Func<string, Project
                     if (options.AllSolutions)
                         return matchingFiles.Select(p => p.ProjectOrSolution);
 
-                    var firstIsClosest = matchingFiles.Length > 1 && matchingFiles[0].DistanceFromRoot < matchingFiles[1].DistanceFromRoot;
-                    if (matchingFiles.Length == 1 || firstIsClosest)
-                        return matchingFiles.Select(p => p.ProjectOrSolution).Take(1);
-
-                    var candidates = matchingFiles.
+                    return matchingFiles.
                         Where(f => f.DistanceFromRoot == matchingFiles[0].DistanceFromRoot).
                         Select(f => f.ProjectOrSolution);
-                    Log(Severity.Info, $"Found multiple '{extension}' files, giving up: {string.Join(", ", candidates)}.");
-                    return new IProjectOrSolution[0];
                 }
 
                 // First look for `.proj` files