Do not generate new models for readonly schema references

Created by: mrginglymus

The changes in #12348 meant that $refs in allOf schemas generated new model names. However, a common pattern is to have a referenced schema with the only modification being readOnly, which is a change to the parent not the referenced schema. In order to not generate duplicate models, special case this one special case.

A demonstration of the problematic model has been included as a typescript-fetch client.

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