Make macro dependencies alias-safe

[danieleades]

Summary

• route all built-in generated-code dependencies through hidden typify re-exports when using import_types!
• resolve the downstream Cargo name for typify, including dependency aliases
• configure Serde derives with the re-exported crate path
• gate macro-only dependencies behind the macro feature and disable that feature for generator-only workspace consumers
• add a downstream compile fixture that aliases typify and declares no peer dependencies

Context

This follows the approach proposed by @jeffs in #1009 and makes it comprehensive. That PR handled regress (only) this version applies the same policy to chrono, regress, serde, serde_json, and uuid, while also accounting for Cargo dependency aliases and Serde derive expansion.

Generated code previously used hard-coded absolute crate paths. Macro consumers therefore needed direct dependencies on Typify's implementation crates, and routing through ::typify alone would fail when the package was imported under a different name.

Standalone TypeSpace, build.rs, and cargo-typify output retains the existing direct crate paths.

User impact

Consumers of typify::import_types! only need to declare typify for built-in generated types. Renaming that dependency in Cargo.toml is supported. User-specified replacement types, conversions, and additional derives still require their own dependencies.