Fix up package indexing and add runtime caching

This patch contains various improvements and fixes that allow much
faster lookup of installed packages. In detail:

• Compress the package index for smaller files, this leads to about 10% improvement in reading, we're trading i/o wait for cpu in here
• Add a runtime cache that remembers the list of packages for 20 seconds during startup of a process. This saves some time on Plasma startup since the list of installed packages is looked up more than once in a short time-frame.
• Save a few strings by sharing the index file name
• Dynamically look up paths to index
• Correct a path printed by kpackagetool

Reviewed-by: Marco Martin <notmart@gmail.com>