Since #16923, - stdin paths are suddenly only supported on the RequirementsSource::Extensionless. However, parsing of cli arguments using from_requirements_txt, from_constraints_txt from_overrides_txt would always output a RequirementsSource::RequirementsTxt. Resulting in the error:

$ cat overrides.txt | cargo run --bin uv --profile dev --manifest-path ./uv/crates/uv/Cargo.toml pip install 'numpy' --overrides=-
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.22s
     Running `./uv/target/debug/uv pip install 'numpy' --overrides=-`
error: File not found: `-`

In this PR, I've added a small check in those for the - paths to use RequirementsSource::ExtensionLess.

I'm not too sure about this change though, as it would also implicitly start allowing PEP 723 scripts as input to overrides/constraints. I don't see the direct issue in that, but then maybe we should explicitly handle it so that an --overrides=script.py would also be supported. @zanieb what do you think?

Relates to #17227