Add ‘path-io’ package

This package provides well-typed interface to ‘directory’ package for
users of Chris Done's ‘path’. It also implements some missing stuff like
recursive scanning and copying of directories.

build-constraints.yaml

@@ -1622,6 +1622,7 @@ packages:
         - megaparsec
         - htaglib
         - slug
+        - path-io
 
     "Thomas Bereknyei <tomberek@gmail.com>":
         - multiplate