Reworked embedded resource enumeration

When enumerating manifest resource names with Linq, Mono throws `InvalidProgramExceptions`. See https://travis-ci.org/asbjornu/GitVersion/builds/132266378#L7579 for details on the failure. Doing a simple `foreach` and `yield return` seems to solve the problem.

Author: asbjornu

src/GitVersionCore/VersionAssemblyInfoResources/AssemblyVersionInfoTemplates.cs

@@ -1,24 +1,18 @@
 namespace GitVersion.VersionAssemblyInfoResources
 {
+    using System;
     using System.Collections.Generic;
     using System.IO;
     using System.Linq;
     using GitVersionCore.Extensions;
 
     public class AssemblyVersionInfoTemplates
     {
-        static IDictionary<string, FileInfo> assemblyInfoSourceList;
+        static readonly IDictionary<string, FileInfo> assemblyInfoSourceList;
 
         static AssemblyVersionInfoTemplates()
         {
-            var enclosingNamespace = typeof(AssemblyVersionInfoTemplates).Namespace;
-
-            var files = typeof(AssemblyVersionInfoTemplates)
-                .Assembly
-                .GetManifestResourceNames()
-                .Where(n => n.StartsWith(enclosingNamespace ?? string.Empty)).Select(f => new FileInfo(f));
-
-            assemblyInfoSourceList = files.ToDictionary(k => k.Extension, v => v);
+            assemblyInfoSourceList = GetEmbeddedVersionAssemblyFiles().ToDictionary(k => k.Extension, v => v);
         }
 
         public static string GetAssemblyInfoTemplateFor(string assemblyInfoFile)
@@ -34,5 +28,19 @@ public static string GetAssemblyInfoTemplateFor(string assemblyInfoFile)
             }
             return null;
         }
+
+        private static IEnumerable<FileInfo> GetEmbeddedVersionAssemblyFiles()
+        {
+            var enclosingNamespace = typeof(AssemblyVersionInfoTemplates).Namespace;
+
+            if (enclosingNamespace == null)
+                throw new InvalidOperationException("The AssemblyVersionInfoTemplates class is missing its namespace.");
+
+            foreach (var name in typeof(AssemblyVersionInfoTemplates).Assembly.GetManifestResourceNames())
+            {
+                if (name.StartsWith(enclosingNamespace))
+                    yield return new FileInfo(name);
+            }
+        }
     }
 }