deduplicate: Avoid traversing directories twice.

Until now, we'd call (nar-sha256 file) unconditionally.  Thus, if FILE
was a directory, we would traverse it for no reason, and then call
'deduplicate' on FILE, which would again traverse it.

This change also removes redundant (mkdir-p store) calls from the loop,
and avoids 'lstat' calls by using 'scandir*'.

* guix/store/deduplication.scm (deduplicate): Add named loop.  Move
'mkdir-p' outside the loop.  Use 'scandir*' instead of 'scandir'.  Do
not call 'nar-sha256' when FILE has type 'directory.