[6.x] Load installed starter kit config files into the running app

[duncanmcclean]

This pull request fixes an issue where installing a starter kit that ships extra filesystem disks (like Peak's favicons disk) would fail with Disk [favicons] does not have a configured driver. when creating the super user or updating the search index.

This was happening because the starter kit's config files are copied into the app mid-install, but the running process still holds the config it loaded at boot. Saving the first user rebuilds the CP search index, which walks every asset container, and the new container's disk isn't in filesystems.disks yet. The same thing happens when search:update runs after install.

This PR fixes it by loading each installed config/*.php file into the config repository as it's copied, using the same dotted key Laravel derives at boot (eg. config/statamic/search.php becomes statamic.search), so later install steps see the kit's config.

Fixes #9884
Fixes #11972