On Windows, `cd` accepts unquoted paths, but for any calls to `xcopy`,
paths arguments must be double-quoted. This patch surrounds all paths
with double-quotes before passing them to `xcopy`.

This fixes #1478: On Wine (and presumably Windows XP), migrating the dub
package directory from dub 1.7 to dub 1.8/1.9 via `xcopy` failed an
enforce in dub (because `xcopy` quit with nonzero exit code) and thus
prevented even the most simple dub projects from being built.

D WYSIWYG strings (backtick-surrounded strings) allow all characters
unescaped, including double quotes and backslashes.