Fix rustdoc crate name extraction

[sfackler]

emberian/rustdoc_ng#15 never got filed here when rustdoc-ng was pulled in.

Rustdoc currently requires #[link(name="foo")] to get the crate name, even through link attributes are being deprecated (#8523). Removing rustdoc as a separate executable entirely and moving the functionality to rustpkg doc seems like the best solution to me.

[alexcrichton]

Nominating. This is a serious hindrance to "just using" rustdoc and rustpkg de-emphasizes the name of the crate in an attribute as well.

[brson]

I'm skeptical about combining rustdoc and rustpkg since they are mostly independent functionality and we don't like creating monolithic super-crates.

[brson]

My preference might be to make rustpkg link to rustdoc.

[emberian]

They need to link to each other. rustdoc needs to know a crate's name, rustpkg needs to be able to doc a crate. Currently it is impossible to use rustdoc with rustpkg.

[emberian]

Fixed. They both use crate_id.