Make private runtime-only dependencies on .NET Standard 2.0. (#11109)

### Context

Since #6148 we produce only reference assemblies for .NET Standard 2.0,
which means that several of the our NuGet packages' dependencies when
targeting .NET Standard 2.0 are unused.

### Changes Made

The project files for `Microsoft.Build.Framework`, `Utilities.Core` and
`Tasks` were updated to apply `PrivateAssets="all"` to all package
references that are not exposed in the package's public API, when
targeting .NET Standard 2.0.

### Testing

I ran `dotnet pack` on these projects and validated manually that on
.NET Standard 2.0, `Microsoft.Build.Framework` has zero dependencies,
`Utilities.Core` depends only on `Framework`, and `Tasks` depends only
on the previous two.

Because Roslyn [keeps some internal APIs in reference
assemblies](https://github.com/dotnet/roslyn/blob/main/docs/features/refout.md#definition-of-ref-assemblies),
these reference assemblies still reference some assemblies whose
respective package is not depended upon. I manually validated with ILSpy
that the types in these assemblies are used only by internal APIs.

### Notes

This is going to be a (minor) source-breaking change if a .NET Standard
2.0 project uses APIs from on one of the removed packages and
transitively depended on it. They will have to directly depend on them,
and it's not the first time we do a change like this (#9055).

I don't think that this is a binary-breaking change because the .NET
Standard 2.0 binaries are not being used at runtime.

---------

Co-authored-by: Jan Provazník <janprovaznik@microsoft.com>

Author: teo-tsirpanis

src/Framework/Microsoft.Build.Framework.csproj

@@ -35,9 +35,10 @@
     <PackageReference Include="System.Diagnostics.DiagnosticSource" />
   </ItemGroup>
 
+  <!-- When targeting NS2.0, make private all references not exposed in the public API. -->
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETStandard'">
-    <!-- Need Win32 API on .NET Standard to ping registry for some methods in ToolLocationHelper -->
-    <PackageReference Include="Microsoft.Win32.Registry" />
+    <PackageReference Update="System.Collections.Immutable" PrivateAssets="all" />
+    <PackageReference Update="System.Diagnostics.DiagnosticSource" PrivateAssets="all" />
   </ItemGroup>
 
   <ItemGroup>

src/Tasks/Microsoft.Build.Tasks.csproj

@@ -696,12 +696,6 @@
     <Content Include="$(NuGetPackageRoot)microsoft.net.compilers.toolset\$(MicrosoftNetCompilersToolsetVersion)\tasks\net472\**\*" CopyToOutputDirectory="PreserveNewest" LinkBase="Roslyn" />
   </ItemGroup>
 
-  <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETStandard'">
-    <PackageReference Include="Microsoft.Win32.Registry" />
-    <PackageReference Include="System.Reflection.Metadata" />
-    <PackageReference Include="System.Threading.Channels" />
-  </ItemGroup>
-
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' != '.NETFramework'">
     <PackageReference Include="System.CodeDom" />
     <PackageReference Include="System.Security.Cryptography.Pkcs" />
@@ -710,6 +704,20 @@
     <Content Include="$(NuGetPackageRoot)microsoft.net.compilers.toolset\$(MicrosoftNetCompilersToolsetVersion)\tasks\netcore\**\*" CopyToOutputDirectory="PreserveNewest" LinkBase="Roslyn" />
   </ItemGroup>
 
+  <!-- When targeting NS2.0, make private all references not exposed in the public API. -->
+  <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETStandard'">
+    <ProjectReference Update="..\StringTools\StringTools.csproj" PrivateAssets="all" />
+    <PackageReference Update="System.CodeDom" PrivateAssets="all" />
+    <PackageReference Update="System.Collections.Immutable" PrivateAssets="all" />
+    <PackageReference Update="System.Resources.Extensions" PrivateAssets="all" />
+    <PackageReference Update="System.Security.Cryptography.Pkcs" PrivateAssets="all" />
+    <PackageReference Update="System.Security.Cryptography.Xml" PrivateAssets="all" />
+    <PackageReference Update="System.Threading.Tasks.Dataflow" PrivateAssets="all" />
+    <PackageReference Include="Microsoft.Win32.Registry" PrivateAssets="all" />
+    <PackageReference Include="System.Reflection.Metadata" PrivateAssets="all" />
+    <PackageReference Include="System.Threading.Channels" PrivateAssets="all" />
+  </ItemGroup>
+
   <ItemGroup>
     <!--
       Content items in this project are used to copy files to the output directory but we don't

src/Utilities/Microsoft.Build.Utilities.csproj

@@ -31,8 +31,13 @@
     <PackageReference Include="Microsoft.VisualStudio.Setup.Configuration.Interop" PrivateAssets="all" />
   </ItemGroup>
 
+  <!-- When targeting NS2.0, make private all references not exposed in the public API. -->
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETStandard'">
-    <PackageReference Include="System.Text.Encoding.CodePages" />
+    <ProjectReference Update="..\StringTools\StringTools.csproj" PrivateAssets="all" />
+    <PackageReference Update="Microsoft.IO.Redist" PrivateAssets="all" />
+    <PackageReference Update="System.Collections.Immutable" PrivateAssets="all" />
+    <PackageReference Update="System.Configuration.ConfigurationManager" PrivateAssets="all" />
+    <PackageReference Include="Microsoft.Win32.Registry" PrivateAssets="all" />
   </ItemGroup>
 
   <ItemGroup Label="Shared Code">