## Problem
Credentials lived in a top-level `_credentials` namespace, requiring
special
preservation logic in `clear_all()` and a separate key hierarchy from
the
platform data they belong to.
## Solution
Move credentials from `_credentials.<platform>` to
`<platform>._credentials`.
Migrate v1 caches on load, skip underscore-prefixed keys when
enumerating
contest IDs and summaries, and simplify `clear_all()` now that no
special
preservation is needed.
Stacked on #292.
1bc0aa41b6