Audit exports from the spanner client mod

[dbolduc]

We should go through all of the re-exports in google-cloud-spanner's client module. Only one of them is a client.

We tend to have:

• mod client - clients
• mod builder - builders for RPC requests, and the client builder.
• mod model - all of the generated message types
• mod model_ext - handwritten message types that are fairly basic
• mod error - any custom errors specific to this crate
• mod foo some complicated thing foo. Like having a mod result_set is probably justified

We don't have any hard and fast rules about when to mod foo vs. when to add foo to mod model_ext.

Originally posted by @dbolduc in #5429 (comment)