Parse `SimpleJson` into categorized data in the client (#522)
Extends #517 with a suggestion from @konstin to parse the `SimpleJson`
into an intermediate type `SimpleMetadata(BTreeMap<Version,
VersionFiles>)` before converting to a `VersionMap`. This reduces the
number of times we need to parse the response. Additionally, we cache
the parsed response now instead of `SimpleJson`.
`VersionFiles` stores two vectors with
`WheelFilename`/`SourceDistFilename` and `File` tuples. These can be
iterated over together or separately. A new enum `DistFilename` was
added to capture the `SourceDistFilename` and `WheelFilename` variants
allowing iteration over both vectors.
