Go modules in conflict with wuffRoot

Recently, go module was introduced, which sounds like the way to go (pardon the pun) with golang.

However this is in conflict with wuff's design, which assumes some source artifacts are available through `$GOPATH/src/github.com/wuffs`.

For example, `wuffs gen` is [hardcoded](https://github.com/google/wuffs/blob/master/cmd/wuffs/gen.go#L145) to consume files from wuffs source root.

A more reasonable design would be not to depend on the wuffs source dir in the modules era, but to package required resources in the executable, and accept path for the sources to generate.

Even if `wuffsRoot` is still deemed necessary, I think we should explicitly copy the required resources to `$GOPATH/src` in `build-all.sh`, in order to have a clean copy of cloned `wuffs` simply work.