@StephenKitt thank you. I found a solution but it's long-winded. Find all the packages which have been implemented in c using
debtags search "implemented-in::c"
send the output to another file or stdout. Take any of those packages, download them via
apt-get source $PACKAGENAME
and then extract from the original.tar.gz and then you know which files are in .c . Admittedly this is not scalable unless content indices for source packages are born sometime in this century. —
shirish 19 mins ago