Warn about ^0.x dependencies (#669)

Warn about ^0.x dependencies (#669)

* Update documentation

* Warn when adding a ^0.x dependency

* Warn on suspicious caret use

This warning is shown at most once per run, and is an optional blocker during
`alr with` and `alr publish`.

A new `Alire.Warnings` package allows simple emission of warnings only once.

* Improve comment wording

* Document user-visible changes

* Rebase fixes

* Make the Warning Id its own type