Warn when using `uv_build` settings without `uv_build` (#15750)
To help with cases such as https://github.com/astral-sh/uv/issues/15655.
A question is when to show this warning. I've used sources as a proxy as
URL dependencies with enabled sources are likely those controlled by the
user, and they include workspace and `git clone`d path dependencies.
The first commit is a refactoring, the second commit the implementation.
Fixes https://github.com/astral-sh/uv/issues/15740
