generate.sh: Add --pkg-prefix for thriftrw invocations
generate.sh: Add --pkg-prefix for thriftrw invocations

Since we no longer have a guarantee that the YARPC source is present
under GOPATH/src/go.uber.org/yarpc, we can't rely on ThriftRW to
automatically determine the target import path.

This change adds the `--pkg-prefix` argument to all invocations of
ThriftRW since ThriftRW can no longer determine this automatically.

yarpc/yarpc-go,yarpc/yarpc-go