Replace FixturePreparer.visit() with list()

The visit() method was dropped in favor of a list() method as the
latter makes it easier to treat fixtureFactory, simpleFixturePreparer
and compositeFixturePreparer as simple collections of already flattened
and deduped *simpleFixturePreparers. That will be used in a follow up
change.

Bug: 183235980
Test: m nothing
Change-Id: Ia41ae9bd4f9c39c85865ba698e0da6be00519caf