Automatically determine which modules have source

Currently, build of ccan is controlled by two Make variables: MODS_WITH_SRC
and MODS_NO_SRC which list modules containing .c files, and modules with
only .h files respectively.

When adding new modules this is fiddly to get right, and there are a number
of modules already listed in the wrong variable.  There's also some
redundant logic in the DIRS variable to again filter out modules without
source.

This simplifies things by having a single manually updated MODS variable
listing every module, and determining MODS_WITH_SOURCE programmatically.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>