Support generation of relative path in JS source maps in JPS builder

See KT-20820

jps-plugin/src/org/jetbrains/kotlin/jps/build/KotlinBuilder.kt

@@ -648,10 +648,21 @@ class KotlinBuilder : ModuleLevelBuilder(BuilderCategory.SOURCE_PROCESSOR) {
         val compilerSettings = JpsKotlinCompilerSettings.getCompilerSettings(representativeModule)
         val k2JsArguments = JpsKotlinCompilerSettings.getK2JsCompilerArguments(representativeModule)
 
-        val sourceRoots = representativeModule.contentRootsList.urls
-                .map { URI.create(it) }
-                .filter { it.scheme == "file" }
-                .map { File(it.path) }
+        // Compiler starts to produce path relative to base dirs in source maps if at least one statement is true:
+        // 1) base dirs are specified;
+        // 2) prefix is specified (i.e. non-empty)
+        // Otherwise compiler produces paths relative to source maps location.
+        // We don't have UI to configure base dirs, but we have UI to configure prefix.
+        // If prefix is not specified (empty) in UI, we want to produce paths relative to source maps location
+        val sourceRoots = if (k2JsArguments.sourceMapPrefix.isNullOrBlank()) {
+            emptyList()
+        }
+        else {
+            representativeModule.contentRootsList.urls
+                    .map { URI.create(it) }
+                    .filter { it.scheme == "file" }
+                    .map { File(it.path) }
+        }
 
         val friendPaths = KotlinBuilderModuleScriptGenerator.getProductionModulesWhichInternalsAreVisible(representativeTarget).mapNotNull {
             val file = getOutputMetaFile(it, false)