v263

• Fixed cache handling so that it now also discards the cache when the package manager (or its version) changes. (#1679)
• Improved the build log output shown when restoring or discarding the cache. For example, if the cache was invalidated all reasons are now shown. (#1679)
• Stopped performing unnecessary cache file copies when the cache is due to be invalidated. This required moving the cache restoration step to after the bin/pre_compile hook runs. (#1679)
• Fixed cache restoration in the case where an app's requirements.txt was formerly a symlink. (#1679)
• Added buildpack metrics for the status of the cache and duration of cache restoration/saving. (#1679)