[REQ][rust] Use relative paths in generated code

Is your feature request related to a problem? Please describe.

The rust generator should support dropping the generated code into an arbitrary path in the crate. Currently, each generated file refers to the absolute path crate::models, forcing the use of hacks like search and replace if the generated could should be in crate::api::models, for example.

Describe the solution you'd like

Rust doesn't make this obvious but we can import the parent without the absolute path using:

use super::super::models;

Start at the Rust generator entry point and trace how generated files emit imports of crate::models; inspect the relevant generation logic or templates. Done means generated code can be placed under an arbitrary crate path such as crate::api::models without search-and-replace, with coverage verifying the relative imports.