Warn when using `uv_build` settings without `uv_build`

[konstin]

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 cloned path dependencies.

The first commit is a refactoring, the second commit the implementation.

Fixes https://github.com/astral-sh/uv/issues/15740