Separate methods used for fixture based and legacy tests

The fixture mechanism makes it easy to refactor by splitting up an
existing preparer into separate ones and then combining them back
together. Unfortunately, that becomes slightly more tricky when
preparers and legacy tests use the same functions to register build
components and define default modules.

This change splits the RegisterRequiredBuildComponentsForTest and
GatherRequiredDepsForTest methods into two methods each, with the
existing method used for legacy tests and calling the new method that
is used for the preparer.

At the moment all the functionality is in the new methods but over
time, as functionality is extracted into separate preparers, the
functionality can also be moved from the method that is common to both
legacy and fixture based tests into the legacy only method.

Bug: 177892522
Test: m nothing
Change-Id: I233a4fe1fb072a00292acc2bb20821ec82a9bd67