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.