Pathological case: the code generator is a target in the generated package

We should detect this edge case and report an informative error to the user.
The code generator must always be defined in a separate package from the generated crate.

No file or test is named in the issue. Trace the code-generation package validation and the handling of the generated crate, then add coverage for the case where the generator is defined in the generated package. Done means this configuration is rejected with an informative user-facing error.

Written by the indexing model from the issue text.