pkg-stats: statistics about license files information

Since on some packages we are adding <pkg>_LICENSE but not necessarily
<pkg>_LICENSE_FILES, let's add a separate statistic to track these
informations. This will allow us to improve both the number of
packages covered by <pkg>_LICENSE and <pkg>_LICENSE_FILES.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>