Add fork_from to repository config

Express a fork as an optional fork_from key on the repository yaml, so a
forked repo is declared the same way as any other repo.

- add fork_from "<owner>/<repo>" to the repository module and object type
- create gitea_fork instead of gitea_repository when fork_from is set
- validate the source repo name matches the config file name, since a fork
  inherits the source name
- move gitea_repository.this to index 0 for the count refactor
- index existing repository import targets to match

modules/gitea_instance/main.tf

@@ -19,6 +19,7 @@ module "repository" {
 
   name                    = each.value.name
   organisation            = each.value.organisation
+  fork_from               = each.value.fork_from
   description             = each.value.description
   private                 = each.value.private
   default_branch          = each.value.default_branch