python packagisation: Rename stbt to stbt.sh, but only in source dir

Rename the "stbt" executable (and the "stbt.in" it's generated from) to
"stbt.sh" (and "stbt.sh.in"). But still install "stbt.sh" as
"$prefix/bin/stbt".

We need to rename it in the source directory so that it doesn't clash
with the python package (added in the next commit) that is a directory
called "stbt".
python packagisation: Move stbt.py to stbt/__init__.py

This is the first step in converting stbt from a python module into a
python package. I've only moved stbt.py to stbt/__init__.py; to make
rebasing easier I'll make any necessary changes in separate commits.
python packagisation: Fix relative paths in stbt/__init__.py

In the previous commit I moved stbt.py (unchanged) to stbt/__init__.py.
Now I'm fixing the relative paths affected by the move.
python packagisation: Move gst_hacks.py into stbt package
python packagisation: Move pylint plugin into stbt package
python packagisation: Move irnetbox.py into stbt package

I considered leaving this outside the stbt package, as it could in
theory be distributed separately to stbt. However my goal with creating
this python package is that I can install it to the system python path,
so that you can import it from anywhere (not just from scripts running
with the "stbt-run" executable, or having to set PYTHONPATH to
$prefix/libexecdir/stbt). But having the stbt package on the system path
is no good if it can't do "import irnetbox". So to simplify
installation, irnetbox.py is going into the stbt package. If someone
wants to pick it up and package it separately, stb-tester's license
allows them to do so.
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._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: Move stbt.get_config etc to stbt.config sub-package