Support requirements without an extension

[charliermarsh]

Summary

This PR un-reverts #16861 by resolving extensionless sources with a different strategy: we return an Extensionless variant, then infer the type when we read the file and parse the contents immediately after, thereby avoiding multiple reads.

[codspeed-hq]

CodSpeed Performance Report

Merging #16889 will degrade performances by 28.44%

_{Comparing charlie/extentionless (7554291) with charlie/source-cache (44e8062)¹}

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	785.4 ms	1,097.4 ms	-28.44%

Footnotes

1. No successful run was found on charlie/source-cache (d5dad4e) during the generation of this report, so d63e56f was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[konstin]

We should special case the error message, asking the user to specify a format manually, as we defaulted to requirements.txt and that failed.

$ uv pip install -r .editorconfig 
error: Couldn't parse requirement in `.editorconfig` at position 102
  Caused by: no such comparison operator "=", must be one of ~= == != <= >= < > ===
root = true
     ^^^^^^

$ uv pip install -r uv.lock
error: Couldn't parse requirement in `uv.lock` at position 0
  Caused by: no such comparison operator "=", must be one of ~= == != <= >= < > ===
version = 1
        ^^^

[zanieb]

There's not a way for them to specify the format though, right?

[konstin]

We use the filename only, but currently there's no indication that this fails because we're trying to parse anything that doesn't look like a pylock.toml as requirements.txt

[zanieb]

I think the hint would just be "We tried to parse this as a requirements.txt because it doesn't have an supported extension"?

For uv.lock, see #16192

[konstin]

That would be a good hint