steps

sbt version: 1.3.3

project/plugins.sbt :
addSbtPlugin("ch.epfl.scala" % "sbt-missinglink" % "0.2.0")

updateSbtClassifiers

problem

plugin sources are not downloaded
idea import does not download plugin sources as well

expectation

plugin sources are available in idea

notes

seems to be related to earlier regressions like #4856