Change rustdoc's `--emit` to allow only one instance of each type,
regardless of the actual data that `--emit` carries.
This matches rustc's `--emit` behavior.
As of the writing, only `dep-info` emit type carries extra data.
See <https://github.com/rust-lang/rust/issues/141664>
rustdoc has many flags to pass external HTML/Markdown/CSS files that end
up in the build. These need to be recorded in depinfo so that Cargo will
rebuild the crate if they change.
