Remove unused dependency to MS.CA.Collections from Framework and …

…surface dependency to immutable collections.

This lightens the NS2.0 reference assembly.

src/Framework/Microsoft.Build.Framework.csproj

@@ -13,17 +13,19 @@
     <EnablePackageValidation>true</EnablePackageValidation>
   </PropertyGroup>
 
-  <ItemGroup>
-    <PackageReference Include="Microsoft.CodeAnalysis.Collections" PrivateAssets="all" />
-  </ItemGroup>
-
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework'">
     <!-- Promote CompilerServices.Unsafe from the old version we get from System.Memory on net472. -->
+    <PackageReference Include="System.Collections.Immutable" />
     <PackageReference Include="System.Runtime.CompilerServices.Unsafe" />
     <PackageReference Include="Microsoft.VisualStudio.Setup.Configuration.Interop" PrivateAssets="all" />
     <Reference Include="System.Xaml" />
   </ItemGroup>
 
+  <!-- When targeting NS2.0, make private all references not exposed in the public API. -->
+  <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETStandard'">
+    <PackageReference Include="System.Collections.Immutable" PrivateAssets="all" />
+  </ItemGroup>
+
   <ItemGroup>
     <Compile Include="..\Shared\Constants.cs">
       <Link>Shared\Constants.cs</Link>