Binary api computes the exported API of a set of Go packages.
The run program is invoked via "go run" from src/run.bash or src/run.bat conditionally builds and runs the cmd/api tool.
TODO(bradfitz): the "conditional" condition is always true. We should only do this if the user has the hg codereview extension enabled and verifies that the go.tools subrepo is checked out with a suitably recently version. In prep for the cmd/api rewrite.