build-std: explicit dependencies

[davidtwco]

Allow users to add explicit dependencies on standard library crates in the Cargo.toml. This enables Cargo to determine which standard library crates are required by the crate graph without build-std-crates being set and for different crates to require different standard library crates.

This RFC is is part of the build-std project goal and a series of build-std RFCs:

1. build-std context (rust-lang/rfcs#3873)
2. build-std="always" (rust-lang/rfcs#3874)
3. Explicit standard library dependencies (this RFC)
4. build-std="compatible" (RFC not opened yet)
5. build-std="match-profile" (RFC not opened yet)

There is also a Zulip channel where you can ask questions about any of the build-std RFCs. This series of RFCs was drafted over many months with the help of stakeholders from many Rust project teams, we thank them for their help!

There are some details that have been noted as being worth mentioning in any eventual tracking issues:

• https://github.com/rust-lang/rfcs/pull/3875#discussion_r2482474212

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