fs: cache non-symlinks in realpathSync.

Extend `fs.realpathSync` to cache the results for paths that are not
symlinks in addition to caching symlink mappings.

PR-URL: #10253
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/fs.js

@@ -1662,6 +1662,7 @@ fs.realpathSync = function realpathSync(p, options) {
       var stat = fs.lstatSync(base);
       if (!stat.isSymbolicLink()) {
         knownHard[base] = true;
+        if (cache) cache.set(base, base);
         continue;
       }