Auto merge of #3899 - alexcrichton:vendor-empty-ok, r=matklad

Support vendor dirs with "empty" directories

Looks like when vendor directories are checked into a VCS there's been instances
where deleting a folder in the VCS doesn't fully delete the folder from the
filesystem. This can lead to [spurious errors][moz] that are difficult to debug.

To help handle this tweak directory sources to ignore empty directories or
directories with only dotfiles by default.

[moz]: https://bugzilla.mozilla.org/show_bug.cgi?id=1342292