Unknown sections are silently ignored for virtual manifest

[RalfJung]

For a virtual manifest (no [package] section), unknown sections in the Cargo.toml sections are silently ignored. For non-virtual packages, however, they trigger a warning.

Behavior should be consistent, and probably also trigger a warning for virtual manifests.

Also see rust-lang/rfcs#1133 (comment).

[Ericson2314]

I'd like all unrecognized data to cause an error---failing fast during configuring is a better user experience than mysterious failure during building. After all, we don't like rustc skip language features it doesn't understand!

Plugins for declarative build.rs replacements can declare what non-standard sections they parse, analogous to the plan for attribute-like procedural macros in Rust.

[ehuss]

This was fixed in #5726.