Auto merge of #9574 - ehuss:weak-namespaced, r=alexcrichton

Auto merge of #9574 - ehuss:weak-namespaced, r=alexcrichton

Unify weak and namespaced features.

This unifies weak and namespaced features in order to simplify the syntax and semantics.  Previously there were four different ways to specify the feature of a dependency:

* `package-name/feature-name` — Enables feature `package-name` on self and enables `feature-name` on the dependency. (Today's behavior.)
* `package-name?/feature-name` — Only enables `feature-name` on the given package if it that package is enabled and will also activates a feature named `package-name` (which must be defined implicitly or explicitly).
* `dep:package-name/feature-name` — Enables dependency `package-name`, and enables `feature-name` on that dependency. This does NOT enable a feature named "package-name".
* `dep:package-name?/feature-name` — Only enables `feature-name` on the given package if it that package is enabled.  This does NOT enable a feature named "package-name".

This changes it so there are only two:

* `package-name/feature-name` — Today's behavior.
* `package-name?/feature-name` — Only enables `feature-name` on the given package if it that package is enabled.  This does NOT enable a feature named "package-name" (the same behavior as `dep:package-name?/feature-name` above).

This is a fairly subtle change, and in most cases probably won't be noticed.  However, it simplifies things which helps with writing documentation and explaining how it works.