Fix serialization issues

Fix `PHP Notice: unserialize(): Error at offset ...` errors.

These kind of errors are caused most of the time by:
- unescaped quotes/slashes etc
- miscounted unicode characters

The cache files will now no longer be human readable. To still be able to identify them, the new cache filenames now include the package slug.

This also takes care of compatibility issues when upgrading as the new version will look for the cache files with the new filename pattern and will disregard the old cache files.