sstate.bbclass: count the files on mirrors using the pre local files

We don't need extra python collections to count the found files
on the sstate cache and sstate mirrors.

The main found collections provides all the files that were found.
Then we only need to store the nunber of files that found on the
local sstate cache and the files that found on the sstate cache
mirror is derived from that.

Signed-off-by: Jose Quaresma <quaresma.jose@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>