Complete support for variants

[stuhood]

The native engine currently has a few stubbings that prevent Variants from working (marked with TODOs mentioning this ticket):

1. Variant merging is implemented by core::Variants, but not used.
2. Requesting the Variants product is implemented by nodes::Select, but not merging them (in particular: this will either require lifting the user-defined variants into rust for merging, or pushing down rust's variants to python for merging)
3. nodes::SelectDependencies does not parse Variants from Address objects (the new path/to:target@key=value syntax) like we used to:

   pants/src/python/pants/engine/nodes.py

   Lines 345 to 350 in ffc0907

   	for dependency in getattr(dep_product, self.field or 'dependencies'):
   	variants = self.variants
   	if isinstance(dependency, Address):
   	# If a subject has literal variants for particular dependencies, they win over all else.
   	dependency, literal_variants = parse_variants(dependency)
   	variants = Variants.merge(variants, literal_variants)

---

Additionally, it has recently occurred to me that the RuleGraph gives us the ability to statically determine whether there are any consumers of Variants in a subgraph. This would allow us to eliminate multiple inefficiencies from the original implementation:

1. We can know statically whether a Variant will be consumed in a subgraph, and so we can avoid propagating Variants in subgraphs that don't consume them (which avoids duplicating Nodes with unused Variant values).
2. Rather than eagerly computing "default" Variant values at every Select Node (which was removed as part of this ticket), we can compute them only in subgraphs where they might be consumed.

[stuhood]

cc @baroquebobcat

[stuhood]

See #5788, which talks about a challenging/interesting usecase for Variants.

[stuhood]

Resolving in favor of #5788.