Update fuzz code to the most recent for module support

The go-fuzz project seems to have started using `golang.org/x/tools/packages` at some point within the last month which means that it should support modules in the correct circumstances for building the binary.

I think it would be good to experiment with this to see if we can make using the fuzzer more user friendly. Right now, it involves me using a special incantation that's in my personal shell history and isn't documented so this issue will be resolved by writing documentation of how to run the fuzzer locally.