Refactor GIRepository GIR generation to avoid cyclical dependency

We're planning to bring g-ir-compiler into this repo but the compiler needs to be built before it can generate the GIR for GIRepository. The current configuration results in a cyclical dependency in this case.

To break the cyclical dependency between girepository and the compiler we need to move generating introspection (introspection/) and any tests which depend on it into girepository/.

This allows ordering the build like:

• libgirepository
• introspection
• tests

Originally raised in !3796 (comment 1955627) from !3796 (closed)