Rationalise / stdise paths vs settings

Per the long discussion here - https://github.com/civicrm/civicrm-core/pull/14718 we have some new 'paths' you can set for urls but others are in 'settings' which is a bit confusing. Ideally we would convert the settings ones but make them overridable via 'settings' as expressed

"Perhaps it's possible to first use Civi::paths behind the scenes and have it populate/override paths set in $civicrm_settings?"