JS: generate paths in source maps relative to .map file location

See KT-19818

compiler/testData/cli/js/sourceMapRelativeRoot.args

@@ -0,0 +1,14 @@
+$TESTDATA_DIR$/sourceMapRelativeRoot/lib/src/lib.kt
+-no-stdlib
+-source-map
+-meta-info
+-output
+$TESTDATA_DIR$/sourceMapRelativeRoot/lib/out/lib.js
+---
+$TESTDATA_DIR$/sourceMapRelativeRoot/main/src/main.kt
+-no-stdlib
+-libraries
+$TESTDATA_DIR$/sourceMapRelativeRoot/lib/out/
+-source-map
+-output
+$TESTDATA_DIR$/sourceMapRelativeRoot/main/out/main.js