How to specify the source code folder for external plugins?

I've got a simple project which uses sbt-idea-plugin and depends on an external plugin - Scala:

intellijBuild in ThisBuild := "2019.3.3"
intellijExternalPlugins += "org.intellij.scala:2019.3.720:Nightly".toPlugin
intellijDownloadSources in ThisBuild := true

sbt-idea-plugin downloaded an SDK with some source code for IntelliJ platform to /home/my-plugin/sdk/2019.3.3/sources.zip and I had to manually specify this zip archive so that the IDE would pick up the source code. Unfortunately, I can't access the source code for the Scala plugin which I depend on.
How do I find and specify the source code for the external plugins my plugin depends on inside the IDE?