code: @Nullable
    static SourceFile getRelativePath(String baseFilePath, String relativePath) {
      return SourceFile.fromPath(
          FileSystems.getDefault().getPath(baseFilePath).resolveSibling(relativePath).normalize(),
          UTF_8);
    }
docs: Returns the relative path, resolved relative to the base path, where the base path is
  interpreted as a filename rather than a directory. E.g.: getRelativeTo("../foo/bar.js",
  "baz/bam/qux.js") --> "baz/foo/bar.js"