[DRAFT] Update cargo_metadata to 0.20 and cargo-platform to 0.2

[musicinmybrain]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

Update cargo_metadata to 0.20 and cargo-platform to 0.2, the last release compatible with the chosen MSRV of 1.85.

It looks like these can be further updated to cargo_metadata 0.23 and cargo-platform 0.3, the latest releases, without additional source-code changes, if the MSRV is advanced to 1.86.

My goal here is to update the packaged cargo-deny in Fedora without having to maintain a rust-cargo_metadata0.19 compat package indefinitely.

The cargo_metadata project doesn’t really keep detailed changelogs, but the changes that required source-code changes can be reviewed in oli-obk/cargo_metadata@0.19.2...0.20.0.

I made a reasonable effort to adapt to the changes in the cargo_metadata API sensibly, but I am not familiar with krates, and I have more experience packaging Rust code than writing it, so review would be appreciated.

I tested this with cargo test --all-features.

Related Issues

N/A

[musicinmybrain]

Oops, this fails when the metadata feature is not enabled, with:

error[E0609]: no field `repr` on type `&std::string::String`
    --> src/builder.rs:1251:54
     |
1251 | ...                   let dsrc = &dsrc.repr;
     |                                        ^^^^ unknown field

That suggests that “peephole” adaptation to cargo_metadata API changes is insufficient, and I need to take a step back and develop a broad understanding of types from cargo_metadata are used in krates’s data structures. I don’t think I’ll have time for that, so I’m going to abandon this PR and proceed with a rust-cargo_metadata0.19 compat package in Fedora for the time being.