Generated source directories not autodetected by IntelliJ

[johanandren]

Even the generated sources directories are not always picked up correctly.

Originally posted by @ennru in https://github.com/lightbend/kalix-jvm-sdk/issues/383#issuecomment-1104882576

Let's make this a separate issue from the one about protobuf dependencies not being found by IntelliJ. I think this problem could possibly be our maven plugins not marking up what generated source directories will be used correctly.

[aludwiko]

Recently, I've noticed that Intellij is very slow in refreshing the sources after a change in proto. My usual workaround is to run "Reload from Disk" action, but that's not very convenient, and it doesn't work every time.