wic: renamd direct_plugin.py -> direct.py

As this files is located in plugins/imager subdirectory it's
obvious that it's an imager plugin. Renamed to direct.py to
be consistent with plugin naming scheme.

[YOCTO #10619]

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>

diff --git a/scripts/lib/wic/plugins/imager/direct_plugin.py b/scripts/lib/wic/plugins/imager/direct.py
similarity index 100%
rename from scripts/lib/wic/plugins/imager/direct_plugin.py
rename to scripts/lib/wic/plugins/imager/direct.py