Prevent uv export from overwriting pyproject.toml

[terror]

Currently, it's possible for uv export to overwrite someones pyproject.toml. This diff simply rejects project files passed in with -o, so we avoid doing that.

[konstin]

What about this (pseudocode):

Suggested change

	"`pyproject.toml` is reserved for project metadata; `{}` exports produce requirements files or `pylock.toml` lockfiles. Choose a different output filename.",
	"`pyproject.toml` is not a supported output format. Supported formats: {}", ExportFormat::value_variants()
	.iter()
	.filter_map(|variant| variant.to_possible_value())
	.map(|value| value.get_name())
	.join(", ");

[terror]

Yeah I like this, just pushed up a change.

[terror]

@konstin Just pushed up a change to placate clippy.

[codspeed-hq]

CodSpeed Performance Report

Merging #16745 will degrade performances by 29.69%

_{Comparing terror:prevent-export-overwrite (8396e22) with main (d3a9455)}

Summary

❌ 1 regression
✅ 5 untouched

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Mode	Benchmark	BASE	HEAD	Change
❌	Simulation	resolve_warm_airflow	795.8 ms	1,131.9 ms	-29.69%