Add early mutators

The mutators that run after dependencies are resolved can be too late
to support build logic that needs to vary the dependencies based on
the mutated axis, for example architecture.  This patch provides an
EarlyMutator interface that can be used to mutate modules before
any dependencies have been resolved.

In order for dependencies to be satisifed in a later pass, all
dependencies of a module must either have an identical variant,
must have a single variant, or must be inserted using
DynamicDependencyModuleContext.AddVariantDependency.

Change-Id: Ic6ae57e98edfd6c8c09a7788983128d3e4e992f0