Update CoreLib sources path and import runtime (dotnet/coreclr#27858)

Commit migrated from https://github.com/dotnet/coreclr/commit/1f2dda30f315b7047f4ec8a5276d888d82f06941

src/coreclr/Directory.Build.props

 <Project>
 
+  <!-- TODO: Consolidate when moved to runtime repository: dotnet/corefx#42170 -->
+  <PropertyGroup>
+    <IsRuntimeRepository Condition="Exists('$(MSBuildThisFileDirectory)..\..\.dotnet-runtime-placeholder')">true</IsRuntimeRepository>
+    <SkipImportArcadeSdkFromRoot>true</SkipImportArcadeSdkFromRoot>
+  </PropertyGroup>
+  <Import Project="..\..\Directory.Build.props" Condition="'$(IsRuntimeRepository)' == 'true'" />
+
   <!-- Ensure our properties are set before Arcade defines defaults -->
   <Import Project="dir.common.props" />
   <Import Project="Sdk.props" Sdk="Microsoft.DotNet.Arcade.Sdk" />

src/coreclr/Directory.Build.targets

 <?xml version="1.0" encoding="utf-8"?>
 <Project>
+  <!-- TODO: Consolidate when moved to runtime repository: dotnet/corefx#42170 -->
+  <Import Project="..\..\Directory.Build.targets" Condition="'$(IsRuntimeRepository)' == 'true'" />
+
   <Import Project="Sdk.targets" Sdk="Microsoft.DotNet.Arcade.Sdk" />
 
   <PropertyGroup>

src/coreclr/src/System.Private.CoreLib/System.Private.CoreLib.csproj

@@ -125,7 +125,9 @@
   <PropertyGroup>
     <CommonPath>$(MSBuildThisFileDirectory)Common</CommonPath>
     <BclSourcesRoot>$(MSBuildThisFileDirectory)src</BclSourcesRoot>
-    <SharedBclSourcesRoot>$(MSBuildThisFileDirectory)shared</SharedBclSourcesRoot>
+    <!-- TODO: Consolidate when moved to runtime repository: dotnet/corefx#42170 -->
+    <SharedBclSourcesRoot Condition="'$(IsRuntimeRepository)' != 'true'">$(MSBuildThisFileDirectory)shared</SharedBclSourcesRoot>
+    <SharedBclSourcesRoot Condition="'$(IsRuntimeRepository)' == 'true'">$(RepoRoot)src\libraries\System.Private.CoreLib\src</SharedBclSourcesRoot>
   </PropertyGroup>
 
   <!-- Msbuild variables needed to get CoreCLR features to be set properly. -->
@@ -414,7 +416,7 @@
     <Compile Include="$(CommonPath)\NotImplemented.cs" />
     <Compile Include="$(CommonPath)\System\SR.cs" />
   </ItemGroup>
-  <Import Project="shared\System.Private.CoreLib.Shared.projitems" />
+  <Import Project="$(SharedBclSourcesRoot)\System.Private.CoreLib.Shared.projitems" />
   <PropertyGroup>
     <CheckCDefines Condition="'$(CheckCDefines)'==''">true</CheckCDefines>
   </PropertyGroup>