artifacts: Add copy dependencies for all artifacts that reference variants

Merged Dduvall requested to merge review/fix-builders-dependencies into main

Any configuration that references build artifacts from another variant should create a cross-variant dependency. Prior to this change, only copies and builder.requirements did so; node.requirements, python.requirements, php.requirements, and builders.*.requirements did not.

Defined a config.VariantDependent interface that builder types can implement to return their cross-variant dependencies.

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