Support namespaced and weak dependency features

[taiki-e]

refs: rust-lang/cargo#10269

concrete example:

• "dep:dependency" features are not running with --feature-powerset and without --optional-deps #153

[taiki-e]

Namespaced features are supported in 0.5.15 (#154). As far as I have tested locally, it seems that weak dependency features are already supported in 0.5.15 and no additional action is needed.