Too many external applications fail to touch the parent directory.
Also removes FP_CACHE_ITEM lastMod times and dirty flags as we've
always loaded libraries atomically anyway.
Claws back some of the performance lost by being more efficient
with cache management for sequential calls to Enumerate and then
Load.
Fixes: lp:1750936
* https://bugs.launchpad.net/kicad/+bug/1750936