Restructure contract files

BepInEx/BepInEx.csproj

@@ -40,7 +40,7 @@
     </Reference>
   </ItemGroup>
   <ItemGroup>
-    <Compile Include="Attributes.cs" />
+    <Compile Include="Contract\Attributes.cs" />
     <Compile Include="Bootstrap\AssemblyPatcher.cs" />
     <Compile Include="Bootstrap\Preloader.cs" />
     <Compile Include="Config.cs" />
@@ -54,7 +54,7 @@
     <Compile Include="ConsoleUtil\Kon.cs" />
     <Compile Include="ConsoleUtil\SafeConsole.cs" />
     <Compile Include="Bootstrap\Chainloader.cs" />
-    <Compile Include="BaseUnityPlugin.cs" />
+    <Compile Include="Contract\BaseUnityPlugin.cs" />
     <Compile Include="Logger.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Bootstrap\TypeLoader.cs" />