Add ref for folders that are missing one

[danmoseley]

Some libraries in corefx have a src folder but no ref folder. In this case the src surface area will be used directly as the ref. This has some disadvantages such as

1. Ref folder is nice for quickly looking at the surface area and its history
2. Ref reduces the size of the references package
3. Possibly a build race, per Figure out build process for libraries with no reference assembly #19584.

We should be consistent and have a ref folder. We already did a pass in #19584. We should do that again and this time ideally add a build step so that new libraries must have one.

These look like they need a ref:

System.Composition
System.Composition.AttributedModel
System.Composition.Convention
System.Composition.Hosting
System.Composition.Runtime
System.Composition.TypedParts
System.Text.Encodings.Web

These do not have one but I believe they produce code that is in a ref in another project (might need special casing in the build step)

System.Private.DataContractSerialization
System.Private.Reflection.Metadata.Ecma335
System.Private.Uri
System.Private.Xml
System.Private.Xml.Linq

Not sure about these, I assume not

Microsoft.Diagnostics.Tracing.EventSource.Redist
Microsoft.IO.Redist
Microsoft.XmlSerializer.Generator

cc @ericstj @weshaggard

[ViktorHofer]

I think the extra build step isn't necessary here as I would be surprised if we allow new libraries without refs in PR reviews. I think the remaining work is to just create the refs for the mentioned projects. Will move the label to enhancement as I don't see any infra work necessarily to be done.