change CookbookSource to set location to a CachedLocation when cached_cookbook is set.
Added CachedLocation as sub class of PathLocation
fixes #718
removing CachedLocation code as it was causing problems.
Resolver shouldn't validate_cached when the location is nil
#718
Merge pull request #772 from b-dean/2-0-stable

Fix issue where location is nil for cookbook that is in the cache