'Stable de-duplication of the directories where the files reside.
>>> l = LibraryList([\'/dir1/liba.a\', \'/dir2/libb.a\', \'/dir1/libc.a\'])
>>> l.directories
[\'/dir1\', \'/dir2\']
>>> h = HeaderList([\'/dir1/a.h\', \'/dir1/b.h\', \'/dir2/c.h\'])
>>> h.directories
[\'/dir1\', \'/dir2\']
Returns:
list of strings: A list of directories'

@property
def directories(self):

    return list(dedupe((os.path.dirname(x) for x in self.files if os.path.dirname(x))))
