[LL FIR] Remove LLFirProviderHelper.getTopLevelClassNamesInPackage

- We shouldn't bypass the symbol names cache, as the IDE declaration
  provider doesn't cache name sets.

^KT-57314

analysis/low-level-api-fir/src/org/jetbrains/kotlin/analysis/low/level/api/fir/providers/LLFirProvider.kt

@@ -101,7 +101,11 @@ internal class LLFirProvider(
 
     override fun getFirFilesByPackage(fqName: FqName): List<FirFile> = error("Should not be called in FIR IDE")
 
-    override fun getClassNamesInPackage(fqName: FqName): Set<Name> = providerHelper.getTopLevelClassNamesInPackage(fqName)
+    override fun getClassNamesInPackage(fqName: FqName): Set<Name> =
+        providerHelper.symbolNameCache.getTopLevelClassifierNamesInPackage(fqName)
+            ?: errorWithAttachment("Cannot compute the set of class names in the given package") {
+                withEntry("packageFqName", fqName.asString())
+            }
 
     @NoMutableState
     internal inner class SymbolProvider : LLFirKotlinSymbolProvider(session) {

analysis/low-level-api-fir/src/org/jetbrains/kotlin/analysis/low/level/api/fir/providers/LLFirProviderHelper.kt

@@ -119,11 +119,6 @@ internal class LLFirProviderHelper(
         return classifierByClassId.getNotNullValueForNotNullContext(classId, classLikeDeclaration)
     }
 
-    fun getTopLevelClassNamesInPackage(packageFqName: FqName): Set<Name> {
-        if (!allowKotlinPackage && packageFqName.isKotlinPackage()) return emptySet()
-        return declarationProvider.getTopLevelKotlinClassLikeDeclarationNamesInPackage(packageFqName)
-    }
-
     fun getTopLevelCallableSymbols(packageFqName: FqName, name: Name): List<FirCallableSymbol<*>> {
         if (!allowKotlinPackage && packageFqName.isKotlinPackage()) return emptyList()
         val callableId = CallableId(packageFqName, name)