Mark referenced packages in SDK as implicitly defined

Should improve package manager behavior
and fix errors in solutions using CPM.

https://github.com/NuGet/Home/issues/13529
https://github.com/dotnet/sdk/pull/43151/files#r1746148520

modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Sdk/Sdk.targets

@@ -25,13 +25,13 @@
 
   <!-- C# source generators -->
   <ItemGroup Condition=" '$(DisableImplicitGodotGeneratorReferences)' != 'true' ">
-    <PackageReference Include="Godot.SourceGenerators" Version="$(PackageVersion_Godot_SourceGenerators)" />
+    <PackageReference Include="Godot.SourceGenerators" IsImplicitlyDefined="true" Version="$(PackageVersion_Godot_SourceGenerators)" />
   </ItemGroup>
 
   <!-- Godot API references -->
   <ItemGroup Condition=" '$(DisableImplicitGodotSharpReferences)' != 'true' ">
-    <PackageReference Include="GodotSharp" Version="$(PackageVersion_GodotSharp)" />
-    <PackageReference Include="GodotSharpEditor" Version="$(PackageVersion_GodotSharp)" Condition=" '$(Configuration)' == 'Debug' " />
+    <PackageReference Include="GodotSharp" IsImplicitlyDefined="true" Version="$(PackageVersion_GodotSharp)" />
+    <PackageReference Include="GodotSharpEditor" IsImplicitlyDefined="true" Version="$(PackageVersion_GodotSharp)" Condition=" '$(Configuration)' == 'Debug' " />
   </ItemGroup>
 
   <!-- iOS-specific build targets -->