Generation of non-file targets in output

[BoPeng]

I have not yet found an example where a step produces usable non-file target. We have

• executable, possible, but even if we can generate an executable in a step, it is not necessarily in $PATH so it still may not exist to sos.
• R_Library and Py_Module: Possible but cannot imagine building a module from a SoS step. Besides, these targets can try to install themselves.
• system_resources: no way to change system resource to make it available.
• sos_step : no need to generate
• sos_variable: use shared to generate, unless we want to change the syntax to output: sos_variable('var'), which might actually work.

So the concept of output: non-file-target seems to be useless. However, in the future we might have

• named_pipe() which creates a pipe that other steps can read,
• s3_bucket to produce a S3 bucket from AWS?
• socket to pass something?

These are possible but at this point I cannot imagine how they should work.