Make the Python plugin provider a libtool module

The Python plugin provider is currently built as a library with an ABI
number and installed in the libdir. It is however not a library, it does
not provide a public API, no program should link to it and we load it
with g_module which is basically a wrapper over dlopen.

As such, build it as libtool "module" installed in the "plugin-providers"
subdirectory which is not part of the library search path.

Add an env variable 'LIBBABELTRACE2_PLUGIN_PROVIDER_DIR' that if is set
will override the builtin path to load the provider.

Change-Id: If475a89733c21f87d6a647e281326c13532a3d3d
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1682
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>