Support dependencies between variants

Variants of a module may need to depend on other variants, for example
to reuse results between variants.  Support AddInterVariantDependency to
add an explicit dependency from one variant to another, along with a
dependency tag.  Both modules must have just been returned by a call to
CreateVariants.

Change-Id: I8f4878f94ced74dd00cfac8303b15ef70cdebf36