patch 9.1.0568: Cannot expand paths from 'cdpath' setting

Problem:  Cannot expand paths from 'cdpath' setting
          (Daniel Hahler)
Solution: Implement 'cdpath' completion, add the new 'dir_in_path'
          completion type (LemonBoy)

fixes #374
closes: #15205

Signed-off-by: LemonBoy <thatlemon@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/builtin.txt

@@ -4065,6 +4065,7 @@ getcompletion({pat}, {type} [, {filtered}])		*getcompletion()*
 		customlist,{func} custom completion, defined via {func}
 		diff_buffer     |:diffget| and |:diffput| completion
 		dir		directory names
+		dir_in_path	directory names in |'cdpath'|
 		environment	environment variable names
 		event		autocommand events
 		expression	Vim expression