Cannot import project to Intellij IDEA after the backend integration as a submodule

[biboudis]

This is probably related to IntelliJ as it re-appears every now and then in IDEA's track. Although I haven't found a solution (apart from git reset --hard <previous commit> and then git merging with dotty's master) I am putting it here as a reminder to update our Building Dotty with Intellij IDEA doc (in case we don't find one).

[smarter]

Ping @jastice: There seems to be an issue with IntelliJ importing sbt projects which use unmanagedSourceDirectories, I've tried to minimize it before, but my minimized projects always succeed the import. Could you help us diagnose what's going on here?

[smarter]

To reproduce the issue:

1. git clone --recursive https://github.com/lampepfl/dotty.git
2. Try to import the project in IntelliJ