Move the function to request and unrequest reviewers to a different
function. This will allow later simplification of the function
that determines if all reviews are complete.
Unrequesting of reviews is only possible in case of bot issued
review requests. The rest are left as-is.
If project or package maintainer already reviewed the PR, as
appropriate, they are no longer re-added to the PR. We also need
to remove reviewers, but only if they were previously requested
by the bot and not something else.
