Make gen.go-thrift skippable

[baroquebobcat]

go-thrift apparently can generate invalid source code. It would be handy to be able to skip generating it by default.

More broadly, I think whether all tasks should be skippable or not is an interesting question to resolve.

[stuhood]

I think that the whole "skip" concept is going to be fundamentally different in the new engine: skipping nodes out of the middle of a graph will just never make sense, so this would become more a question of root selection. So I think I'm just going to link this to #4769 and resolve it.