Implement candidate index

This provides cabal with the means to conveniently opt into accessing package release candidates, by presenting them as a secondary package index to cabal. This secondary package index would have lesser guarantees (this follows from the properties/nature of package candidates; specifically, .cabal revisions are not supported, instead package releases can be deleted, or be mutated destructively in-place). The primary and candidate index are comparable to Debian's "stable" and "testing"/"experimental" distributions respectively; package releases start in the transient candidate index, and can be promoted to the eternal/monotonic primary index if they pass basic validations.

TODO: specify design in more detail

The issue provides no files, tests, or entry points to begin with. First specify the candidate-index design, including its relationship to the primary index and package promotion rules. Done means the design is detailed enough to guide implementation and validation.

Written by the indexing model from the issue text.