Hidden directories are now ignored.

A hidden directory on most Posix file systems starts with a period. e.g.
`.dub`. By default, dub ignored hidden files (e.g. `.swap.file.d`), but not
hidden directories. Some operating systems create hidden directories that dub
would try to include for compilation. This release now will properly ignore
hidden directories, unless those directories are specifically named in the dub
recipe file.

Note that this uses the directory name exclusively and does not check file
attributes.