We let the get_deps() caller hand us the compiled _info filename, but what
about recursive dependencies?  These we re-generate every time.

So fix this: hand a generator callback to get_deps(), expose the one
which simply compiles it, and add a ccanlint one which looks up the
manifest to see if we have one already.

Before:
$ ccanlint -vvvv ccan/failtest | grep -c 'Creating.*_info'
31

After:
$ ccanlint -vvvv ccan/failtest | grep -c 'Creating.*_info'
17