When the classpath references a unpackaged (not a jar) directory, scaladoc does not generate links to the url provided via the -doc-external-doc flag.

Pretty sure this fixes it:
jgrean/scala@becc362

But I need to figure out the testing+PR process, and verify I've used the canonical way to verify a class file is part of a classpath, before I'm ready to submit a PR.

Example Case:
We have two separate scala libraries we're building from source in the same sbt parent project, with nightly scaladoc builds. Currently it's a pain to link them without first packaging the dependency as a jar and referencing that in the dependent project instead of the sbt project.