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>