(maint) rename prompts.rb to prompt.rb

To bring this inline with some other utilties, I'm changing this from plural to
singular form. Every method in this submodule will be a different user prompt
on STDIN.
(maint) Move 'ask_yes_or_no' into Pkg::Util::Prompt

Pkg::Util::Prompt.ask_yes_or_no now has some formatting
wrapped into it, and it strips (not just chomps) responses.
It also shows the user their invalid input if they answer
anything other than yes or no to the question.
(maint) More refactoring to keep us from shipping on anything but a tag

I've cleaned up numerous helper methods under lib/packaging/util and
revised the logical flow between notifications, prompts, and validations.

- Validations should only confirm a condition, or otherwise fail.

- Notifications should only relay information across stdout or stderr
(they should not change the execution flow of the program).

- Prompts should relay some basic amount of information and ask a user
to take action based on the information relayed.

These are the general workflow for many of the rake tasks we run (take a repo,
confirm user intent to do something with it, do the thing that we said we would)
so I've tried to construct basic methods that can be recycled as we tease apart
the numerous rake tasks.