refactor(compare-command): Move an enum classs closer to its use

While the `enum class` does not need command context, a nested class in
Kotlin does not have outer class context anyway (only `inner` classes
have), so it does not hurt to scope the enum to the command where it is
being used.

Signed-off-by: Sebastian Schuberth <sebastian@doubleopen.org>

plugins/commands/compare/src/main/kotlin/CompareCommand.kt

@@ -51,6 +51,8 @@ class CompareCommand : OrtCommand(
     name = "compare",
     help = "Compare two ORT results with various methods."
 ) {
+    private enum class CompareMethod { SEMANTIC_DIFF, TEXT_DIFF }
+
     private val fileA by argument(help = "The first ORT result file to compare.")
         .convert { it.expandTilde() }
         .file(mustExist = true, canBeFile = true, canBeDir = false, mustBeWritable = false, mustBeReadable = true)
@@ -166,11 +168,6 @@ class CompareCommand : OrtCommand(
     }
 }
 
-private enum class CompareMethod {
-    SEMANTIC_DIFF,
-    TEXT_DIFF
-}
-
 private class EpochInstantDeserializer : StdDeserializer<Instant>(Instant::class.java) {
     override fun deserialize(parser: JsonParser, context: DeserializationContext): Instant =
         Instant.EPOCH.also {