util/disk_cache: rename mesa cache dir and introduce cache versioning

util/disk_cache: rename mesa cache dir and introduce cache versioning

Steam is already analysing cache items, unfortunatly we did not
introduce a versioning mechanism for identifying structural changes
to cache entries earlier so the only way to do so is to rename the
cache directory.

Since we are renaming it we take the opportunity to give the directory
a more meaningful name.

Adding a version field to the header of cache entries will help us to
avoid having to rename the directory in future. Please note this is
versioning for the internal structure of the entries as defined in
disk_cache.{c,h} as opposed to the structure of the data provided to
the disk cache by the GLSL compiler and the various driver backends.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>