K2: Optimize AbstractFirDeserializedSymbolProvider

Avoid filling caches with keys that are definitely empty
(if it's cheap to compute that), to decrease the size of backing maps.

The strategy is pre-computing the sets of names that might be met.
NB: the size of the sets is way fewer than a size of all queried names.

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/session/KlibBasedSymbolProvider.kt

@@ -94,6 +94,11 @@ class KlibBasedSymbolProvider(
         }
     }
 
+    override fun computePackageSetWithNonClassDeclarations(): Set<String> = fragmentNamesInLibraries.keys
+
+    // Looks like it's expensive to compute the presence of a class properly for KLib
+    override fun mayHaveTopLevelClass(classId: ClassId): Boolean = true
+
     @OptIn(SymbolInternals::class)
     override fun extractClassMetadata(classId: ClassId, parentContext: FirDeserializationContext?): ClassMetadataFindResult? {
         val packageStringName = classId.packageFqName.asString()