Move unconventional import rule to post-binding phase (#5151)
## Summary
This PR moves the "unconventional import alias" rule (which enforces,
e.g., that `pandas` is imported as `pd`) to the "dead scopes" phase,
after the main linter pass. This (1) avoids an allocation since we no
longer need to create the qualified name in the linter pass; and (2)
will allow us to autofix it, since we'll have access to all references.
## Test Plan
`cargo test` -- all changes are to ranges (which are improvements IMO).
