LU-18687 contrib: move cc-plugins, clarify license

LU-18687 contrib: move cc-plugins, clarify license

Move cc-plugins from the top-level to contrib. This is
a more sensible location. Also, clarify that it available
under GPLv2 and add Apache-2.0 to LICENSES/dual/ directory.
This mirrors the practices of the Linux kernel.

Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I462b4ded84e9bd78692a87532da3df4f703db302
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57939
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>