Rename CheckError::IndexLookup to CheckError::ApiLookup #4
Description
The CheckError::IndexLookup variant name is a holdover from the sparse index approach. Now that we use the crates.io REST API, it should be renamed to ApiLookup (or ApiError) for clarity.
This is a breaking change to the public API, so it should be batched with the next semver-breaking release.
Context: surfaced during PR #2 code review (switch from sparse index to API).