feat(builtin-deps): Add `SourceKind::Builtin` (#17513)

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### What does this PR try to resolve?

As part of https://github.com/rust-lang/cargo/issues/16960 this PR adds
a new variant of SourceKind to represent Builtin packages and integrates
it into explicit builtin dependencies. This allows resolution to proceed
slightly further, but is now blocked a Source capable of returning
Packages and Summaries for builtin packages.

A few notes:

- A previous version of this PR bumped the version of
`cargo-util-schemas`, but this has already been done this release cycle.
- It's not quite clear what the behaviour of parsing/displaying URLs
related to SourceIds should be, and I'm going to defer that to when I
handle user-facing behaviour that requires it.
- There's improvements possible to the way we fetch the standard library
source path. We have a task on the goal's work plan to handle this later
when the core implementation for Builtins is a bit more settled. For now
I think this solution is reasonable.

### How to test and review this PR?

Commit by commit - all commits pass tests. Existing user-facing
behaviour is limited, but one test is updated to show the difference.

🤖 LLM disclosure: I used Codex to help me understand the codebase and
asked it to review my branch before sharing it upstream. I also used it
in limited ways to generate code, including resolving merge conflicts
and updating test assertions. All code was originally written by hand.

r? @Muscraft (we're trying to share out these reviews a bit, but I don't
mind who reviews)