[FIR] KT-57207 Avoid FirJavaFacade.knownClassNamesInPackage in the IDE

- `FirJavaFacade.knownClassNamesInPackage` cannot be computed in the IDE
  using the current strategy because there are multiple finders and
  there is no `CliFinder`. However, the cache was still used, which
  caused it to be filled with `null` values and additionally caused
  worse performance in `JavaSymbolProvider` due to hash map accesses via
  `hasTopLevelClassOf`.
- Rewriting the strategy is non-trivial as additional indices are needed
  on the IDE side. See KTIJ-24642.

compiler/javac-wrapper/src/org/jetbrains/kotlin/javac/components/JavacBasedClassFinder.kt

@@ -45,4 +45,5 @@ class JavacBasedClassFinder : AbstractJavaClassFinder() {
     override fun findPackage(fqName: FqName, mayHaveAnnotations: Boolean) = javac.findPackage(fqName, javaSearchScope)
 
     override fun knownClassNamesInPackage(packageFqName: FqName) = javac.knownClassNamesInPackage(packageFqName)
+    override fun canComputeKnownClassNamesInPackage(): Boolean = true
 }