packaging: Move all "common" into private package _stbt

With the dockerized tools coming up in stb-tester we want to use some of
the stb-tester functionality without incurring a dependency on the rest.
e.g. stbt service needs get/set_config but doesn't need a dependency on
GStreamer or OpenCV.  This has not been possible as all our code was in
a single `stbt.py` file.  The re-organisation 8ae1c0c split `stbt.py` up
into multiple files, but you still couldn't `import stbt.config` without
incurring a dependency on GStreamer.

This commit splits the stbt package into a private package `_stbt` with
no hard dependencies and a public one `stbt` with all of them.  Our
internal tools can now `import _stbt.config` etc. without hacks to
`PYTHONPATH`, etc to get import resolution to work.  It has the secondary
beneficial effect that our users won't think that `import stbt.config` is
a valid thing to do.  We still only officially support `import stbt`.
packaging: Rename `config._set_config` to `config.set_config`

`set_config` is internal to stbt, not internal to config.py.