[K/JS] Throw error when a module contains file with the same name and package (ignoring case)

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/ModuleFragmentToExternalName.kt

@@ -15,9 +15,24 @@ import org.jetbrains.kotlin.ir.declarations.path
 private const val EXPORTER_FILE_POSTFIX = ".export"
 
 class ModuleFragmentToExternalName(private val jsOutputNamesMapping: Map<IrModuleFragment, String>) {
+    private val externalNameToItsFile = hashMapOf<String, IrFile>()
+
     fun getExternalNameFor(file: IrFile, granularity: JsGenerationGranularity): String {
         assert(granularity == JsGenerationGranularity.PER_FILE) { "This method should be used only for PER_FILE granularity" }
-        return file.module.getJsOutputName().getExternalModuleNameForPerFile(file)
+        return file.module.getJsOutputName().getExternalModuleNameForPerFile(file).also {
+            val alreadyReservedBy = externalNameToItsFile.putIfAbsent(it.lowercase(), file)
+
+            if (alreadyReservedBy != null && alreadyReservedBy != file) {
+                error(
+                    """
+                      |There are two files in module '${file.module.name}' that have the similar package and file names.
+                      |  - Package "${file.packageFqName.asString()}" and path "${file.path}"
+                      |  - Package "${alreadyReservedBy.packageFqName.asString()}" and path "${alreadyReservedBy.path}"
+                      |Note, that if the difference is only in letter cases, it also could lead to a clash of the compiled artifacts
+                   """.trimMargin()
+                )
+            }
+        }
     }
 
     fun getExternalNameForExporterFile(file: IrFile, granularity: JsGenerationGranularity): String {