(maint) Make ignore-existing optional for Pkg::Util::Net.rsync_to

There are occasions where it is desirable for rsync_to to clobber
existing files, such as for updating a latest directory on a nightly
repository. Previously, this behavior wasn't possible without shelling
out to a custom rsync, so this commit adds a parameter to the rsync_to
method to allow ignore-existing to be excluded from the rsync flags. It
defaults to true, to maintain existing behavior as the default.
(CPR-64) Refactor latest linking for nightly repos

This commit updates the nightly repos workflow to maintain a static
$project-latest directory with a linked repos directory. The
repo_configs are generated to have static filenames and contents, to
ensure that users can simply add the repos and have their package
manager keep them up to date with $project's master branch. The repos
symlink in the latest directory is updated for each project when a new
version ships, and this should be totally transparent to users of the
latest symlink.
This also tweaks the rsync out to not use --ignore-existing, as
otherwise the -latest directory is never updated or replaced.
Merge pull request #328 from haus/ticket/master/cpr_64_refactor_latest_work

(CPR-64) Refactor latest linking for nightly repos