Add a list of mods to Skill class

Although this isn't necessary for existing official rulesets and calculators, custom calculators can have use cases for accessing mods in difficulty calculation.
For example, accounting for the effects of visual mods.

osu.Game.Rulesets.Osu/Difficulty/OsuDifficultyCalculator.cs

@@ -79,10 +79,10 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             }
         }
 
-        protected override Skill[] CreateSkills(IBeatmap beatmap) => new Skill[]
+        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods) => new Skill[]
         {
-            new Aim(),
-            new Speed()
+            new Aim(mods),
+            new Speed(mods)
         };
 
         protected override Mod[] DifficultyAdjustmentMods => new Mod[]