Fix "find all references" when using the IDE in the Dotty build

Running "find all references" in Dotty itself used to fail because
constructing the map of project to their dependency failed, because this
map is based on the `projectDependencies` key written in
`.dotty-ide.json` which might contain projects which are not loaded in
the IDE because they were excluded with `excludeFromIDE`.
This commit fixes this by just ignoring dependencies which do not
correspond to projects loaded in the IDE, this is the most resilient
way to fix this.

language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala

@@ -156,7 +156,7 @@ class DottyLanguageServer extends LanguageServer
       val allProjects = drivers.keySet
 
       def transitiveDependencies(config: ProjectConfig): Set[ProjectConfig] = {
-        val dependencies = config.projectDependencies.map(idToConfig).toSet
+        val dependencies = config.projectDependencies.flatMap(idToConfig.get).toSet
         dependencies ++ dependencies.flatMap(transitiveDependencies)
       }