lei inspect: use LeiMailSync->match_imap_url

lei inspect: use LeiMailSync->match_imap_url

Move match_imap_url into LeiMailSync so it can be used in more
places, such as "lei inspect".  Upcoming commands such as
"lei forget-mail-sync" and {add,forget,pause,resume}-watch will
also support relaxed IMAP matching rules since there's
no reasonable way to expect users use ";UIDVALIDITY=" on the
command-line.