fix: handle index.md when checking relative links

Author: yyx990803

lib/webpack/markdownLoader.js

@@ -58,13 +58,19 @@ module.exports = function (src) {
 
   // check if relative links are valid
   links && links.forEach(link => {
-    const filename = link
+    const shortname = link
       .replace(/#[\w-]*$/, '')
       .replace(/\.html$/, '.md')
+    const filename = shortname
       .replace(/\/$/, '/README.md')
       .replace(/^\//, sourceDir + '/')
-    const file = path.resolve(path.dirname(this.resourcePath), filename)
-    if (!fs.existsSync(file)) {
+    const altname = shortname
+      .replace(/\/$/, '/index.md')
+      .replace(/^\//, sourceDir + '/')
+    const dir = path.dirname(this.resourcePath)
+    const file = path.resolve(dir, filename)
+    const altfile = altname !== filename ? path.resolve(dir, altname) : null
+    if (!fs.existsSync(file) && (altfile && !fs.existsSync(altfile))) {
       this.emitWarning(
         new Error(
           `\nFile for relative link "${link}" does not exist.\n` +