Use ReferenceVariantsHelper for code completion

[`ReferenceVariantsHelper`](https://github.com/JetBrains/kotlin/blob/master/idea/ide-common/src/org/jetbrains/kotlin/idea/codeInsight/ReferenceVariantsHelper.kt) bundles a lot of completion logic (e.g. for handling Java getters/setters) that would simplify the language server's completion generation.