Refactor Python agent build and install

Since the Python agent's tracepoint provider,
liblttng-ust-python-agent, does not depend on Python, it can
always be built and installed alongside LTTng-UST.

The Python package of this agent is completely independent
from the rest of the tree, thus it is isolated in its own
directory. This also eases the creation of distribution
packages because the packager can selectively build and
install the Python package without also building/installing the
tracepoint provider.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>