composer: search files also through installed rpm

for some awkward binary we need to search though the rpm DB
yum DB does not have all those information. For example :
/usr/libexec/gcc/x86_64-redhat-linux/4.4.6/collect2
/usr/libexec/gcc/x86_64-redhat-linux/4.4.6/cc1
are found on the yum DB but only if the rpm is installed on the
local machine
composer: do not duplicate self.packages (it's now a set)