Description:
Delegates dependency detection to the child process.
This is done by creating pipes for the child process to send back the
dependency information. The environment variables BUTTON_INPUTS and
BUTTON_OUTPUTS are set to the file descriptors that the child should write
to. This is also useful for the child process to determine if it is running
under this build system or not. The child only needs to check if both of
those environment variables are set.
This handler should be used for commands that know how to communicate with
Button. It is also commonly used by other handlers to run the command.