uv: parse scripts entries and warn on non-package projects

[lucab]

This is a followup to #7034 and #7420.

It would be good to parse the scripting-related tables in pyproject.toml.

That unlocks two UX enhancements:

• uv run can key-match and warn in a selective and non-annoying way.
• uv sync can directly display the script name (or the total number of script, if many) that it skipped.