In prep for Robert's forthcoming cmd/api rewrite which
depends on the go.tools subrepo, we'll need to be more
careful about how and when we run cmd/api.

Rather than implement this policy in both run.bash and
run.bat, this change moves the policy and mechanism into
cmd/api/run.go, which will then evolve.

The plan is in a TODO in run.go.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/12482044