cpp-common: rename `bt2_common` namespace -> `bt2c`

cpp-common: rename `bt2_common` namespace -> `bt2c`

I always end up using the `bt2c` namespace alias in `*.cpp` files
anyway.

Also put all the `bt2c` stuff in `cpp-common/bt2c` to make it clear.

What remains in `cpp-common`, but outside `cpp-common/bt2c`, are the
libbabeltrace2 C++ bindings (`bt2` namespace) and external projects.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ife8527aa961025f28d06707eb9f374cafea572f5
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11390
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>