3.0 constants

Refs #5452, #5451 

This makes Cache packages usable as standalone without requiring constant CACHE. Since it depends on Core package I added DS constant there.

Another package which requires similar treatment is Log. I don't think system temp dir is a good default for logs so better to just remove use of LOGS constant and have user always specify path for FileLog.
crazy, under what criteria are the functions.php files loaded in composer?