rustdoc: Add an --extern flag analagous to rustc's

This adds an `--extern` flag to `rustdoc` much like the compiler's to
specify the path where a given crate can be found.
auto merge of #15822 : tomjakubowski/rust/add-externs-rustdoc, r=alexcrichton

This adds an `--extern` flag to `rustdoc` much like the compiler's to
specify the path where a given crate can be found.