Fetch dependencies recursively

This uses some general make tools, most notably "while" and "select" which come from
https://github.com/markpiffer/gmtt.git and which allow us to recursively parse the
.dep files in the given e3 installation. Broadly speaking, we start with all variables
of the form `%_DEP_VERSION` to determine the first line of dependent modules, and then
we ask them (using `fetch_deps`) what they depend on.