Warn when `tool.uv.build-backend` is set but wrong `[build-system]` is configured

e.g., in https://github.com/astral-sh/uv/issues/15655#issue-3379565490 we don't respect the `build-backend` setting because our build backend isn't being used. We should have warnings for

1. When no build system is configured
2. When the wrong build system is configured
3. When setuptools legacy would be used  (e.g., due to `package = true`)