Add support for module variant aliases

Adding a dependency on a module with variants can be problematic
if the code adding the dependency is not aware of every mutator
that has created variants.  Add an AliasVariations to
BottomUpMutatorContext, which allows a mutator to alias the
original variant of a module to one of the new variants of the
module, which will allow future dependencies to be added using
the original list of variations.  The aliases are transient,
and only exist until the next mutator that calls CreateVariations
when visiting the module without also calling AliasVariations.

Test: TestAlises
Change-Id: Ieaa04b5a6bdcb5a1ff5114b1e03460de795d4479