Add option validation to plugin compiler

[mnonnenmacher]

As it is possible to define aliases for plugin options, there could be overlaps which make it ambiguous which option is meant. For example:

PluginConfig(
  val option1: String,

  @PluginOption(aliases = ["option1"])
  val option2: String
)

In this example it is ambiguous which actual option an "option1" entry in the config map refers to.

Add validation to the plugin compiler that reports an error if any such overlaps exist.

The same problem exists for enum options after #11268 is merged, so similar validation must be added.