GenIdea does not trigger target recomputation

[Baccata]

I found it whilst testing #351, and found that I had to call upon mill __.ivyDeps after adding/removing dependencies to my modules in order for the idea config to be generated appropriately.

Steps to reproduce :

• Generate idea config from mill
• Add/remove an arbitrary ivy dependency to one module
• Regenerate idea config (without running any other mill command after previous step)
• Look at cache in /out/module/ivyDeps to check whether the dependency was indeed added/removed

[raskam]

In my case calling mill __.ivyDeps is not sufficient and I always have to remove out/ and .idea_modules/ directories before running mill mill.scalalib.GenIdea/idea.

I also have to reopen the project in Idea, because sometimes Idea would still use old classpath.