Upgrader implementation ignore UpgraderSkin interfaces

The new Upgrader implementations like Plugin_Upgrader don't stick to the interfaces they claim to use in terms of the WP_Upgrader_Skin and just directly access properties like $overwrite that are only declared for _some_ implementations.

This is a violation of the open/closed principle and cause code that already extends the UpgraderSkin to break. An example of this can be found here: ​https://travis-ci.org/github/wp-cli/wp-cli/jobs/708222192#L505-L511

The code should either first check whether the properties/methods that are not part of the interface are actually available or not, or it should only accept specific implementations, or an adapted interface.