Parse requirements-style inputs into RequirementsInput

[woodruffw]

Summary

This adds a RequirementsInput API, which enumerates the Stdin, Local and Remote states discretely. The idea here is to make some of the current CLI behavior more correct by construction -- instead of parsing into a PathBuf and then trying to sniff what we really have, we parse into a RequirementsInput instead.

Some things to note:

• This does the right default thing, i.e. file://... becomes RequirementsInput::Local instead of Remote with a file: scheme. And - becomes a RequirementsInput::Stdin.
• This applies to --requirements, but also --constraints and any other flag that previously had local and remote requirements-style syntax.
• We propagate RequirementsInput down to the parsing layer, meaning that we're now doing a better job of tracking the provenance of our inputs. This will be relevant when I resume uv audit: pylock.toml support #21601.
• Right now we model - as RequirementsInput::Local, but that seems wrong. We probably want a new RequirementsInput::Stdin.
• Because RequirementsInput::Remote is a DisplaySafeUrl, this also gets us free redaction in a few more places that weren't easy to cover before.

This also makes one behavioral change: we now handle remote nested requirements more correctly. Previously we would handle a subset of these incidentally, e.g.:

uv pip install -r https://example.com/foo/a.txt

and then a.txt:

-r b.txt

This would correctly fetch https://example.com/foo/b.txt.

However, if a.txt was this instead:

-r /b.txt

...we would attempt to load a local requirements file from /b.txt rather than https://example.com/b.txt. Now that requirements loading is more type-aware, we do the join correctly depending on whether it's a URL or a path.

This is super marginal in practice, but now our behavior matches pip's.

Test Plan

Updated the tests, and added new ones.

[woodruffw]

NB d28726f is needed because #21067 added a test case where --find-links should give a local directory named https:links precedence over treatment as a URL.

[astral-sh-bot]

uv test inventory changes

This PR changes the tests when compared with the main base revision.

• Added tests: 3
• Removed tests: 0
• Changed suites: 2

uv::it: +1 / -0

Added:

• uv::it::requirements::parse_requirements_input

Removed: none

uv::pip_install: +2 / -0

Added:

• uv::pip_install::pip_install::install_remote_requirements_txt_redacts_nested_url
• uv::pip_install::pip_install::install_remote_requirements_txt_with_relative_include

Removed: none

[zanieb]

don't you mean to use from?

[woodruffw]

My 0.02c is it's idiomatic when the receiver is impl Into<T> since there's no target ambiguity. @MichaReiser can strike me down though 🙂

[zanieb]

Probably outside this pr, but we're repeating this error clause

[woodruffw]

Yeah, I'll do this as a fast-follow.