Missing Java links in API docs for Scala

[ashawley]

For example, the link to "Java Streams" on this page:

https://www.scala-lang.org/api/2.13.0/scala/jdk/StreamConverters$.html

Should link to the documentation for java.util.stream.Stream.

Building Scala, there are a bunch of warnings when running doc:

[warn] src/library/scala/jdk/StreamConverters.scala:17:1: Could not find any member to link for "java.util.stream.Stream".
[warn] /** This object provides extension methods to create [[java.util.stream.Stream Java Streams]] that
[warn] ^

This could be fixed by adding some setting to sbt, see scala/scala-xml#146

Scaladoc doesn't do the best job with generating links to entries of Javadoc, but it does ok most of the time.

[SethTisue]

perhaps we ought to be using https://github.com/ThoughtWorksInc/sbt-api-mappings ? (ThoughtWorksInc/sbt-api-mappings#6)

[ashawley]

Yes, that would also work. That plug-in was originally meant to help with tricky situations linking to popular Scala libraries and to Scala itself, but later added Javadoc support, as well. I don't think Scala will have any Scala dependencies to cross-reference.

I've added just the lightweight solution in scala/scala#8284.