support/scripts/pkg-stats: import cve module only when needed

support/scripts/pkg-stats: import cve module only when needed

The cve module needs ijson, which may not be installed. Since cve
matching is only enabled when --nvd-path is passed, it is a bit silly
to error out about ijson being missing if it's not used.

So instead of unconditionally importing the cve module, only do it
conditionally.

However, instead of doing it right at the point where it is used, we
do it at the beginning of the main() function. Indeed, if the cve
module is needed but cannot be imported, we want to error out
immediately rather than doing a whole bunch of things, and failing on
the user later on in the middle of the pkg-stats execution.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>