Support uv add -r requirements.txt

[blueraft]

Summary

Resolves #4537

• First commit avoids overwriting dependencies with different markers.
• Second commit supports adding from requirements files.

Test Plan

cargo test

[zanieb]

Should we allow these files? These seem like a bit of a misuse. We don't allow them in uv run

uv/crates/uv/src/commands/project/run.rs

Lines 74 to 92 in 2822dde

	for source in &requirements {
	match source {
	RequirementsSource::PyprojectToml(_) => {
	bail!("Adding requirements from a `pyproject.toml` is not supported in `uv run`");
	}
	RequirementsSource::SetupPy(_) => {
	bail!("Adding requirements from a `setup.py` is not supported in `uv run`");
	}
	RequirementsSource::SetupCfg(_) => {
	bail!("Adding requirements from a `setup.cfg` is not supported in `uv run`");
	}
	RequirementsSource::RequirementsTxt(path) => {
	if path == Path::new("-") {
	bail!("Reading requirements from stdin is not supported in `uv run`");
	}
	}
	_ => {}
	}
	}

[blueraft]

Supporting setup.py and setup.cfg files could be a helpful feature, making it easier for users to transition to the pyproject.toml standard.

[charliermarsh]

It's possible that this could be a killer feature because you could import Poetry projects this way... But it doesn't actually work as-is. I'm gonna remove it and look at it in a separate PR.

[zanieb]

Could you separate the change to avoid marker collisions from the requirements.txt support? I think they'll make more sense to discuss in two pull requests.

[charliermarsh]

Assigning myself.