rustdoc fails on packages that use cargo workspaces

[hjr3]

Example:

     Running `target/debug/rustdoc --manifest-path=../habitat/ build`
generating save analysis data...
Problem creating configuration: Cargo failed with status exit code: 101. stderr:
warning: profiles for the non root package will be ignored, specify profiles at the workspace root:
package:   /Users/herman/projects/habitat/components/director/Cargo.toml
workspace: /Users/herman/projects/habitat/Cargo.toml
warning: profiles for the non root package will be ignored, specify profiles at the workspace root:
package:   /Users/herman/projects/habitat/components/sup/Cargo.toml
workspace: /Users/herman/projects/habitat/Cargo.toml
error: manifest path `/Users/herman/projects/habitat/Cargo.toml` is a virtual manifest, but this command requires running against an actual package in this workspace

[euclio]

Noting that I just ran into this when trying to document rustdoc itself. For a non-virtual workspace, it'll just pick the first package in the workspace indiscriminately (rustdoc-ember) and fail.