Reorganize public modules around driver concepts

Current public module layout is based partly on implementation details rather than user-facing concepts.

Examples:

• scylla.enums contains both enums and non-enum configuration objects.
• scylla.types contains only the Unset sentinel.
• scylla.value contains only CqlEmpty.
• scylla.session_builder and scylla.execution_profile create modules around individual implementation classes.
• CQL type descriptors live under scylla.cluster.metadata.
• scylla.policies re-exports many unrelated policy APIs.

This causes several problems:

• Users must know how an object is implemented to find it.
• Related concepts are spread across unrelated modules.
• Generic modules such as enums, types, and value become ambiguous.
• Changing an enum into another representation would unnecessarily affect its import location.
• Empty root exports force verbose and difficult-to-discover imports.

Public modules should describe driver concepts and remain stable even when internal representations change.

Start by mapping the public exports and relationships among scylla.enums, scylla.types, scylla.value, scylla.session_builder, scylla.execution_profile, scylla.cluster.metadata, and scylla.policies. Done means related driver concepts have discoverable public modules, root exports are useful, and imports remain stable when internal representations change.