yang-system-test: -p directories are for dependency search

This request is not very compatible with YANGTOOLS-726 so that would need to use option different from -p.

Examining existing Yang validators, it turns out their focus is not on validating a whole set of Yang models, but on validating a single Yang file instead.
This single file is given as an argument (not -p option).

Of course most Yang modules are including other modules, so to ascertain validity those other modules should also be available. Thus a Yang validator needs to be able to search for dependencies, and -p directories are the locations the validator should search for the needed files in (defaulting to current working directory if no -p is given).

In order for yang-system-test to be used as a Yang validator in existing validation frameworks, it should honor the same semantics.

As a evidence, here [0] is description of -p in yanglint, and here [1] is how yangvalidator.com invokes pyang to validate a particular Yang file.

[0] https://github.com/CESNET/libyang/blob/master/tools/lint/main_ni.c#L50
[1] https://github.com/cmoberg/bottle-yang-extractor-validator/blob/master/bottle-yang-extractor-validator/main.py#L93

P.S.: For purpose of validation of single files, perhaps odl-yang-validator would be a better name than yang-system-test.