Refactor "offline" feature placement in the API

From Karel:

The location of offline() method in MavenStrategyStage unfortunately does not cover all possible use cases:
Imagine the following:
User loads a pom - now the first resolution happens, because an effective pom is created and this is later used to define the dependencies:

final File[] longhand = Resolvers.use(MavenResolverSystem.class).configureFromPom("path/to/pom").resolve("G:A").offline()
            .withTransitivity().as(File.class);

I don't think that delaying ".configureFromPom("path/to/pom").resolve("G:A")" call until MavenStrategyStage is reached in the right way to go, because that way we couldn't for instance cache already parsed POM file.