Consolidate to one standard approach for running WP-CLI commands from within commands

[danielbachhuber]

Currently, we have two different ways of running new WP-CLI commands from within an existing command:

• WP_CLI::launch_self() - launches a new process while reusing some runtime args and env variables, but not all.
• WP_CLI::run_command() - calls the WP-CLI command using the existing process.

It's confusing to use these methods within a WP-CLI command, so I still end up writing my own implementation (e.g. wp-cli/restful@778c3e0#diff-157c802629de9f14607866f958873927R209)

It would be nice to have a WP_CLI::run_command() which:

• Accepted the command as a string (e.g. 'rest post list --format=json'), because this is the most intuitive way for a developer to define the argument. These passed arguments would be automatically shell escaped, so the caller didn't have to worry about it.
• Supported launching a new process, or running the command within the existing process. If a new process was launched, then the method would appropriately recycle our environment variables.
• Either displayed the result of the command (Use proc_open() instead of passthru() to persist TTY mode #3117), or captured and returned the result of the execution.

[aaemnnosttv]

This seems like the way to go, as both launch_self and run_command didn't work for me and I was more/less forced to reimplement this based on what you did in the rest diff command there.