Rename mk2rbc/cmd to make `go install` work properly

The standard go tooling assumes that a package's binary name should be
the name of the containing folder. Before this change, `go install` or
`go build` (without `-o`) names this binary `cmd`.

Test: treehugger
Change-Id: I45a4e97f4a0d59744d644e30636250ceefe3bd1c