[ty] Rename module search traversal operation to `advance` (#28912)
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
In response to feedback on https://github.com/astral-sh/ruff/pull/28201,
this renames the `enter_package` method from `ModuleSearchCursor` to
`advance`. I've factored it out from
https://github.com/astral-sh/ruff/pull/28201 to reduce noise from
trivial changes.
## Test Plan
This is a refactor that relies on existing test coverage.
<!-- How was it tested? -->
ruff
8a38ca2b - [ty] Rename module search traversal operation to `advance` (#28912)
