internal: prepare to move rename to base_db

internal: prepare to move rename to base_db

It's better to handle magical cases upper in the stack, because it
allows for better re-use of the general implementation below. So, we
pull the `self` case up here.

The end goal is to put `Definition::rename` to the `ide_db`, because
it's a generally re-usable functionality useful for different ide
features, alongside with the search which is already there.