CargoWorkspace's package list includes packages that are path
dependencies, even if those packages aren't actually members of the
cargo workspace. As a result, rust-analyzer's runnable finder, which
returns the target from the first workspace that has a matching package,
may select the wrong working directory, causing runnables to fail, e.g.,
```
error: package `root` cannot be tested because it requires dev-dependencies and is not a member of the workspace
```
To fix this, we filter out packages that aren't members of the workspace
when searching for targets.
Fixes#7764
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).
