Moving tests to `rust-analyzer` crate allows removing walkdir dependency
from `xtask`. It does seem more reasonable to keep tidy tests outside of
the "build system" and closer to other integration tests.
* Keep codegen adjacent to the relevant crates.
* Remove codgen deps from xtask, speeding-up from-source installation.
This regresses the release process a bit, as it now needs to run the
tests (and, by extension, compile the code).
