Fix FSharp.Core references in FCS solution (dotnet#13827)

* Allow using FSharp.Core package in new service solution projects

* Remove inner rules

* Another FSharp.Core reference

* Fix relative path

Co-authored-by: Vlad Zarytovskii <vzaritovsky@hotmail.com>
Co-authored-by: Kevin Ransom (msft) <codecutter@hotmail.com>

Author: 3 people

tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj

@@ -216,6 +216,7 @@
 
   <ItemGroup Condition="'$(FSHARPCORE_USE_PACKAGE)' != 'true'">
     <ProjectReference Include="$(MSBuildThisFileDirectory)..\FSharp.Core\FSharp.Core.fsproj" />
+    <ProjectReference Include="$(FSharpSourcesRoot)\FSharp.Core\FSharp.Core.fsproj" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(FSHARPCORE_USE_PACKAGE)' == 'true'">