Make Generator output names predictable (for RunGen usage)

[steven-johnson]

Generators that use Output<> can end up with ${SOME_INTEGER} appended to the actual name of the output, due to unique-ifcation of Funcs.

Generators that use build() don't have any way to specify a name for the outputs, and end up with f{SOME_INTEGER} as the output name.

In both cases, a predictable name would be better: in the former case, we should ensure that the output names match the declared name; in the latter case, we should generate a predictable-but-likely-unique name (e.g. of the form "${GENERATORNAME}_output").