Prefer full names over aliases for crate dependencies

[coryan]

In a number of places we use aliases for crate names, such as gax instead of google-cloud-gax. That saves us typing, but produces examples that may be confusing: where we use gax::error::Error instead of google_cloud_gax::error::Error the reader may wonder what gax is, and may even find a bad crate.

Likewise, the source code buttons at docs.rs are less useful if there are a lot of aliases to keep in ones head.

I removed most aliases without filing a bug (tsck, tsck). But it is time to explain why we are doing this. The remaining crates are google-cloud-gax, google-cloud-gax-internal and google-cloud-wkt.

The outline is:

• Add some pub(crate) use google_cloud_gax::Foo; use declarations to simplify the generated code.
• Generate the code with crate::Foo; everywhere.
• Use google-cloud-gax in storage, pubsub, firestore, and the generated clients.
• Add conditional pub(crate) use google_cloud_gax::Bar; use declarations for pagination.
• Use these declarations in the generated code.
• Add some pub(crate) use google_cloud_gax_internal::Bar; use declarations to simplify the generated code.
• Use crate::Bar everywhere.
• Use google-cloud-gax-internal in storage, pubsub, firestore and the generated clients.
• Add some pub(crate) use google_cloud_wkt::internal::Bar; use declarations to crate::internal.
• Change the generated code to say crate::internal instead of wkt::internal
• Add the minimum set of google_cloud_wkt::* declarations to (a new pub(crate) module) crate::wkt.
• Change the generated code to say crate::wkt instead of wkt.

Keeping all the new symbols pub(crate) would allow changes later, in case we decide that they should all be in a sub-module, or in the root module, or some other alternative.

[coryan]

We still need to fix wkt and gaxi, but I need to stop and work on tracing for storage first.