closes#8056🎨 Collect together the package-related utils
- read directory actually reads a directory of packages
- parse package json is very tighly related to this
🎨 Move filterPaths -> packages.filterPackages
- this function is related to packages, not settings
- move the function to the new utils/packages
- add 100% test coverage
🎨 Simplify filterPackages code
🎨 Simplify reading of packages & themes
- This massively reduces all the complex code in the read packages & themes utils
- Added full test coverage
🎨 Improve & clarify active prop in filterPackages
- active is returned from API endpoints to combine data from multiple sources
- see https://github.com/TryGhost/Ghost/pull/8064#discussion_r103514810🎨 Better error handling
🔥 Temporarily remove custom error templates
- we will reimplement this later when we have got a better concept of loading the active theme in place
- refs #8079
