ToResolverCache should return a Dictionary

fixes

```
        Tasks/LinkAssemblies.cs(103,41): error CS1503: Argument 1: cannot convert from 'System.Collections.IDictionary' to 'System.Collections.Generic.Dictionary<string, Mono.Cecil.AssemblyDefinition>'
```

in xamarin-android

Author: lewurm

src/Java.Interop.Tools.Cecil/Java.Interop.Tools.Cecil/DirectoryAssemblyResolver.cs

@@ -97,13 +97,9 @@ protected virtual void Dispose (bool disposing)
 		}
 
 		[Obsolete ("Should not be used; was required with previous Cecil versions.")]
-		public IDictionary ToResolverCache ()
+		public Dictionary<string, AssemblyDefinition> ToResolverCache ()
 		{
-			var resolver_cache = new Hashtable ();
-			foreach (var pair in cache)
-				resolver_cache.Add (pair.Key, pair.Value);
-
-			return resolver_cache;
+			return new Dictionary<string, AssemblyDefinition>(cache);
 		}
 
 		public virtual AssemblyDefinition Load (string fileName)