Let primary APIs have the better module name (CodableStructuredHeaders -> StructuredHeaders)

As mentioned in the [announcement thread](https://forums.swift.org/t/introducing-swift-http-structured-headers/44041/5), if users are intended to start with the higher level module, `CodableStructuredHeaders`, shouldn't it get the better module name? This increases discoverability (users don't need to know about the other module) and allows users to start at the high level rather than low. The current `StructuredHeaders` could be renamed to `LowLevelStructuredHeaders` or anything else that indicates it's lower level than the other module.

Start by inspecting the package and module declarations, then trace references to StructuredHeaders and CodableStructuredHeaders across the repository. Determine the final public and lower-level names with maintainers; done means the rename is applied consistently and the package still builds and tests pass.

Written by the indexing model from the issue text.