scancpan: handle recommended dependencies as optional packages

scancpan: handle recommended dependencies as optional packages

Currently, without the flag -recommend, scancpan takes as dependency
only one which has the relationship "requires"; this mode works fine.
And, with the flag -recommend, scancpan takes all ones (ie. with
relationship "requires" or "recommends") in the same way; this mode
never works fine, because it is too simplistic.

With this commit, the "not required" dependencies are handled as
optional BR package or skipped if a cyclic dependency is detected.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>