Add dedicated error message for conda environment.yml files

[zanieb]

If used via -r, we should say we don't support these?

See #12596

[zanieb]

I'm not quite sure how we'd detect this, but we could start with an exact name match special case on parse error?

[christeefy]

I can give this a shot, @zanieb.

To confirm, is it correct to say that:

• the changes will happen somewhere within src/lib.rs::run of the uv crate?
• on parse error, raise a dedicated error message if the filename matches environment.yml?

[zanieb]

I don't think it'd be in lib.rs::run, I think there we just parse a PathBuf into a RequirementsSource. It'd be downstream where we are actually reading the requirements files?

I suggested special-casing the parse error because it's the "safest" option, we only fail if we were going to fail anyway. However, it's possible it'd be better to skip that and instead add a RequirementsSource variant EnvironmentYml which we construct when the path matches environment.yml. We'd then error on later, e.g., in RequirementsSpecification::from_source? I'm not sure without trying though.

[christeefy]

Got it! I assumed that RequirementsSource.from_requirements_file would be fallible, but I see now that validation is deferred until RequirementsSpecification::from_source, as you pointed out.

I'll try to implement a fix based on your suggestions, and may circle back here if I need extra clarification.