Encourage the use of explicit input names when specifying dependencies

[schrockn]

For inputs that are dependencies, right now the APIs strongly encourage/enforce that the input name and the solid name should be the same. That's a mistake, I believe. It can cause for very awkward names for transform function arguments. More importantly, someone changing the name of the solid would break any solid that depends on that name.

Best example is dep_only_input. This should take a name.

We also might want to consider an API that instead passes in a dictionary of input specifications where the name is the key. This would enforce uniqueness, etc.