Using custom import during codegen for re-exported crates

Hello, currently code that's generated using `prost-validate-build` always ends up hardcoding `::prost-validate`. This makes it tricky to build a code-generating tool on top of it and re-export `prost-validate`, so the users can only worry about specifying a single crate in the dependencies.

Would you be open to a PR that adds a method such as `prost_validate_path` for using a re-exported `prost-validate` crate?

For example, this is how it can be configured in `prost-build`: https://docs.rs/prost-build/latest/prost_build/struct.Config.html#method.prost_path

Start by tracing prost-validate-build's code-generation entry point and the generated references that hardcode ::prost-validate. Compare the requested configuration with prost-build's Config::prost_path documentation. Done means callers can configure the path for a re-exported crate and generated code uses that path consistently.

Written by the indexing model from the issue text.