This allows us to start building real dependencies into Makefile.
Leave old hack/* scripts in place but advise to use 'make'. There are a few
rules that call things like 'go run' or 'build/*' that I left as-is for now.
Right now some of the hack/* tools use `go run` and build almost every
time. There are some which expect you to have already run `go install`.
And in all cases the pre-commit hook, which runs a full build wouldn't
want to do either, since it just built!
This creates a new hack/after-build/ directory and has the scripts which
REQUIRE that the binary already be built. It doesn't test and complain.
It just fails miserably. Users should not be in this directory. Users
should just use hack/verify-* which will just do the build and then call
the "after-build" version. The pre-commit hook or anything which KNOWS
the binaries have been built can use the fast version.
