python packagisation: Move stbt._mkdir to stbt.utils.mkdir

This is needed to avoid circular imports when I move `stbt._set_config`
to `stbt.config._set_config` in a future commit.
python packagisation: stbt.ConfigurationError is Exception not UITestError

This is to avoid a circular import when I move ConfigurationError to the
"stbt.config" sub-package. This is a breaking change.

ConfigurationError inherits from Exception rather than another more
specific exception, so that we have an upgrade path if in future we
decide to change `get_config` to raise `TypeError` and/or `ValueError`.
python packagisation: Move stbt.get_config etc to stbt.config sub-package