core: copy all sub-directories in a box properly

This avoids an Errno::EACCES on windows, although I'm really unsure why.

CHANGELOG.md

@@ -66,6 +66,7 @@ BUG FIXES:
   - core: Synced folders set to the default synced folder explicitly won't
     be deleted. [GH-2873]
   - core: Static IPs can end in ".1". A warning is now shown. [GH-2914]
+  - core: Adding boxes that have directories in them works on Windows.
   - commands/box: Box add `--force` works with `--provider` flag. [GH-2757]
   - commands/box: Listing boxes with machine-readable output crash is gone.
   - commands/plugin: Plugin installation will fail if dependencies conflict,

lib/vagrant/box_collection.rb

@@ -166,13 +166,27 @@ module Vagrant
             # Move to final destination
             final_dir.mkpath
 
-            # Go through each child and copy them one-by-one. This avoids
-            # an issue where on Windows cross-device directory copies are
-            # failing for some reason. [GH-1424]
-            final_temp_dir.children(true).each do |f|
-              destination = final_dir.join(f.basename)
-              @logger.debug("Moving: #{f} => #{destination}")
-              FileUtils.mv(f, destination)
+            # Recursively move individual files from the temporary directory
+            # to the final location. We do this instead of moving the entire
+            # directory to avoid issues on Windows. [GH-1424]
+            copy_pairs = [[final_temp_dir, final_dir]]
+            while !copy_pairs.empty?
+              from, to = copy_pairs.shift
+              from.children(true).each do |f|
+                dest = to.join(f.basename)
+
+                # We don't copy entire directories, so create the
+                # directory and then add to our list to copy.
+                if f.directory?
+                  dest.mkpath
+                  copy_pairs << [f, dest]
+                  next
+                end
+
+                # Copy the single file
+                @logger.debug("Moving: #{f} => #{dest}")
+                FileUtils.mv(f, dest)
+              end
             end
 
             if opts[:metadata_url]