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

packagefs: Add support for hpkg format V1

* Add union-like class PackageData which wraps the V1 and V2
BPackageData classes.
* GlobalFactory: Create a data reader depending on the data format
version.
* Package: Add a loader for V1 format and try that, if the other one
fails.

packagefs: Organize sources into subdirectories

packagefs: Add support for hpkg format V1

* Add union-like class PackageData which wraps the V1 and V2
BPackageData classes.
* GlobalFactory: Create a data reader depending on the data format
version.
* Package: Add a loader for V1 format and try that, if the other one
fails.

packagefs: Organize sources into subdirectories