Allow private rule attributes ("_something") to be set from wrapper macros declared with the rule.

It's a common idiom to use a wrapper macro (call it "foo") around a rule (call it "_foo") to do things like compute defaults. Sometimes these defaults are for values which we want to compute at load time (e.g. glob() results). We can pass this from the wrapper to the implementation with an attribute named "private_something", but that is not idiomatic. We could enhance readability by allowing `_x` attribute names for things passed solely from the wrapper to the implementation.