support/scripts/pkg-stats: fixed numeral in beginning of CSS class

CSS classes are generated for each package name for sorting purposes,
However some package names start with a number and this is not allowed.
(see https://www.w3.org/TR/CSS2/syndata.html#value-def-identifier

)
Fix is to prepend a character to every class name such as "_".
so every ".package" is now "._package".

Signed-off-by: Sen Hastings <sen@phobosdpl.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>