[mtouch/mmp] Make it possible to compute assembly identity even if an…

… assembly doesn't have a filename.

In .NET AssemblyDefinitions don't have MainModule.FileName set, because they're not
loaded using FileStreams.

Ref: dotnet/linker#1313

tools/common/Assembly.cs

@@ -69,11 +69,13 @@ public string FullPath {
 			}
 		}
 		public string FileName { get { return Path.GetFileName (FullPath); } }
-		public string Identity { get { return GetIdentity (FullPath); } }
+		public string Identity { get { return GetIdentity (AssemblyDefinition); } }
 
 		public static string GetIdentity (AssemblyDefinition ad)
 		{
-			return Path.GetFileNameWithoutExtension (ad.MainModule.FileName);
+			if (!string.IsNullOrEmpty (ad.MainModule.FileName))
+				return Path.GetFileNameWithoutExtension (ad.MainModule.FileName);
+			return ad.Name.Name;
 		}
 
 		public static string GetIdentity (string path)