Ensure repository deletion is consistent (#6214)

This change improves the consistency of `Pool().remove_repository()` to make it easier to write poetry plugins which mutate the repository pool.

1.  Deleting an element from the middle of `Pool()._repositories` decrements the index of later entries.  Update `Pool()._lookup` to reflect this.

2.  If a primary repository is deleted, decrement `Pool()._secondary_start_idx` to ensure that any additional primary repositories are still added before all secondary repositories.

3.  If the default repository is deleted, reset `Pool()._default` so a new one can be added.

Co-authored-by: Randy Döring <30527984+radoering@users.noreply.github.com>