Enable auto-import assist for prelude and glob imported items

Use case

The anyhow crate provides the following definition:

pub type Result<T, E = Error> = core::result::Result<T, E>;

If you create a new file, and use the type Result<()> somewhere, there is no way to automatically add use anyhow::Result;, because Result is already a type that exists.

Proposed solution:

If the item under the cursor has been imported from prelude or a glob import, allow auto-importing an alternative item with the same name.

Provide a new assist for changing the import of the item under the cursor.
If the item is defined in the current module, no assist is shown.
If the item is already imported from a use, allow selecting an alternative import which replaces the old one.
If the item is imported by default, like std's Result, allow selecting an import to hide it.

Reproduce the example with anyhow::Result and inspect the existing auto-import assist and its tests, focusing on how prelude and glob-imported names are resolved. Determine how an alternative same-named item can be offered without ambiguity, then add coverage showing the expected assist behavior for prelude and glob imports.