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/variables.tf

@@ -17,6 +17,7 @@ variable "repository" {
   type = map(object({
     name                    = string
     organisation            = string
+    fork_from               = optional(string)
     description             = optional(string)
     private                 = optional(bool)
     default_branch          = optional(string)