Fix support for archives without directory structure

In case of archives that do not store directories we would fail to automatically serve index.html for a / type of request.

This makes us create directories when traversing the file list.

Archive:  public-without-root-dir.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
       31  2020-09-15 02:47   public/404.html
      258  2020-10-14 22:47   public/bad_symlink.html
       33  2020-09-15 02:47   public/index.html
        0  2020-10-14 22:47   public/subdir/
       40  2020-09-15 02:47   public/subdir/hello.html
       14  2020-09-15 03:35   public/subdir/2bp3Qzs9CCW7cGnxhghdavZ2bJDTzvu2mrj6O8Yqjm3YMRozRZULxBBKzJXCK16GlsvO1GlbCyONf2LTCndJU9cIr5T3PLDN7XnfG00lEmf9DWHPXiAbbi0v8ioSjnoTqdyjELVKuhsGRGxeV9RptLMyGnbpJx1w2uECiUQSHrRVQNuq2xoHLlk30UAmis1EhGXP5kKprzHxuavsKMdT4XRP0d79tie4tjqtfRsP4y60hmNS1vSujrxzhDa
       32  2020-10-14 22:47   public/subdir/index.html
       33  2020-09-15 02:47   public/subdir/linked.html
       18  2020-10-14 22:47   public/symlink.html
---------                     -------
      459                     9 files

Closes #482 (closed)