Address minor code review

BepInEx.Core/Utility.cs

@@ -236,7 +236,7 @@ public static class Utility
 
         var potentialSymbolsFileNames = new[]
         {
-            $"{filename}.mdb",
+            $"{filename}.dll.mdb",
             $"{filename}.pdb"
         };
                 

BepInEx.PatcherProvider/BepInExPatcherLoadContext.cs

@@ -43,5 +43,6 @@ public class BepInExPatcherLoadContext : IPluginLoadContext
     public void Dispose()
     {
         assemblyData = null;
+        assemblySymbolsData = null;
     }
 }

BepInEx.PluginProvider/BepInExPluginLoadContext.cs

@@ -43,5 +43,6 @@ public class BepInExPluginLoadContext : IPluginLoadContext
     public void Dispose()
     {
         assemblyData = null;
+        assemblySymbolsData = null;
     }
 }