util.join() doesn't handle absolute path on Windows correctly

lib/util.js の util.join() と util.isAbsolute() から始め、報告された Windows のドライブレター付きパスを reproducer として使用します。両方のエントリポイントが、F:\Projects\... のようなパスをスラッシュで始まるパスとともにどのように扱うかを確認します。絶対 Windows パスが認識され、source-map ローダーパイプライン全体で繰り返し結合されなくなれば完了です。その後、既存のプロジェクトテストを実行します。

We use the Webpack pipeline on Windows with sass-loader, postcss-loader, and css-loader. Then we found the generated source paths went crazy: F:\Projects\web-project-boilerplate\src\scss\index.scss just becomes F:/Projects/web-project-boilerplate/src/scss/F:/Projects/web-project-boilerplate/src/scss/F:/Projects/web-project-boilerplate/src/scss/index.scss.

As I followed the codebase in the pipeline, I noticed that the sass-loader use relative path, then the postcss-loader and css-loader turn to use absolute path for "sources" field in the sourceMap. In addition, the absolute path on Windows starts with a symbol of the disk driver, e.g. F: in our case. When it comes to sourceMap's util.join(), it only detects absolute path starts with /

https://github.com/mozilla/source-map/blob/c3a8f5a2a4cc7ac5bc6c06bc93745d3c57bd8860/lib/util.js#L228

So it joins multiple paths across the loaders.

Also, the util.isAbsolute doesn't deal with the Windows absolute path as well.

https://github.com/mozilla/source-map/blob/c3a8f5a2a4cc7ac5bc6c06bc93745d3c57bd8860/lib/util.js#L241

Appendix:
Here is a case of how expressjs test the absolute path:

exports.isAbsolute = function(path){
  if ('/' === path[0]) return true;
  if (':' === path[1] && ('\\' === path[2] || '/' === path[2])) return true; // Windows device path
  if ('\\\\' === path.substring(0, 2)) return true; // Microsoft Azure absolute path
};

I'm not sure we need to handle the azure absolute path, but I would be better if we had window device path included.

Thanks