[ty] Run benchmarks for auto-imports in CI (#28490)
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
This adds a few benchmarks for the language server's "auto-import"
feature to the set of benchmarks that we run in CI.
At present, we benchmark auto-imports for a regular package layout in
three Salsa database states: completely fresh (i.e. no cache), for an
identical, repeated request (i.e. fully-cached), and after a module
creation (i.e. partially-cached). This also includes setup for
benchmarking namespace packages, but those benchmarks remain disabled
until such support lands via
https://github.com/astral-sh/ty/issues/2273.
## Test Plan
You can see the results of the initial benchmarks runs
[below](https://github.com/astral-sh/ruff/pull/28490#issuecomment-5621409843).
<!-- How was it tested? -->
