Prefix everything inside Rulesets.Objects.Legacy with "Convert" to avoid naming clashes with Ruleset projects.

osu.Game/Rulesets/Objects/Legacy/Osu/ConvertHit.cs

@@ -9,7 +9,7 @@ namespace osu.Game.Rulesets.Objects.Legacy.Osu
     /// <summary>
     /// Legacy osu! Hit-type, used for parsing Beatmaps.
     /// </summary>
-    internal sealed class Hit : HitObject, IHasPosition, IHasCombo
+    internal sealed class ConvertHit : HitObject, IHasPosition, IHasCombo
     {
         public Vector2 Position { get; set; }
 

osu.Game/Rulesets/Objects/Legacy/Osu/ConvertHitObjectParser.cs

@@ -11,11 +11,11 @@ namespace osu.Game.Rulesets.Objects.Legacy.Osu
     /// <summary>
     /// A HitObjectParser to parse legacy osu! Beatmaps.
     /// </summary>
-    internal class HitObjectParser : Legacy.HitObjectParser
+    internal class ConvertHitObjectParser : Legacy.ConvertHitObjectParser
     {
         protected override HitObject CreateHit(Vector2 position, bool newCombo)
         {
-            return new Hit
+            return new ConvertHit
             {
                 Position = position,
                 NewCombo = newCombo,
@@ -24,7 +24,7 @@ namespace osu.Game.Rulesets.Objects.Legacy.Osu
 
         protected override HitObject CreateSlider(Vector2 position, bool newCombo, List<Vector2> controlPoints, double length, CurveType curveType, int repeatCount, List<List<SampleInfo>> repeatSamples)
         {
-            return new Slider
+            return new ConvertSlider
             {
                 Position = position,
                 NewCombo = newCombo,
@@ -38,7 +38,7 @@ namespace osu.Game.Rulesets.Objects.Legacy.Osu
 
         protected override HitObject CreateSpinner(Vector2 position, double endTime)
         {
-            return new Spinner
+            return new ConvertSpinner
             {
                 Position = position,
                 EndTime = endTime

osu.Game/Rulesets/Objects/Legacy/Osu/ConvertSlider.cs

@@ -9,7 +9,7 @@ namespace osu.Game.Rulesets.Objects.Legacy.Osu
     /// <summary>
     /// Legacy osu! Slider-type, used for parsing Beatmaps.
     /// </summary>
-    internal sealed class Slider : LegacySlider, IHasPosition, IHasCombo
+    internal sealed class ConvertSlider : Legacy.ConvertSlider, IHasPosition, IHasCombo
     {
         public Vector2 Position { get; set; }
 

osu.Game/Rulesets/Objects/Legacy/Osu/ConvertSpinner.cs

@@ -9,7 +9,7 @@ namespace osu.Game.Rulesets.Objects.Legacy.Osu
     /// <summary>
     /// Legacy osu! Spinner-type, used for parsing Beatmaps.
     /// </summary>
-    internal sealed class Spinner : HitObject, IHasEndTime, IHasPosition
+    internal sealed class ConvertSpinner : HitObject, IHasEndTime, IHasPosition
     {
         public double EndTime { get; set; }