packagefs: Use object caches for the most used node classes.

Slab statistics from KDL show that on a stock nightly image
(i.e. no additional packages besides the standards installed)
there are 43848 *objects* (not bytes) in the PackageNodeAttribute
cache, and 25090 in the PackageFile cache, so this seems more than
worth it.

The last commit seems to reduce memory usage at boot by about 1%,
this commit seems to not affect it at all; but it is a significant
performance optimization and on systems with more packages installed
the effect may be very noticeable.

Change-Id: I676a642ed6003f82b14396e1f02684575d899362