Inconsistent package name of "transport"

https://github.com/aws/smithy-go/blob/main/endpoints/endpoint.go

The directory is `endpoint` but the package is `transport`. This violates Go's naming conventions.

Start with the package declaration in endpoints/endpoint.go and search the repository for references to the transport package. Rename the package to match the endpoint directory, update affected references, and confirm the Go checks pass with no remaining naming inconsistency.

Written by the indexing model from the issue text.