Fix provider API to not use `product_query`

[knoepfel]

The API for registering providers with Phlex v0.2.0 looks like:

  g.provide("provide_number", provide_number, concurrency::unlimited)
    .output_product(product_query{.creator = "input", .layer = "run", .suffix = "number"});

where provide_number is a user-defined function that receives a data_cell_index. There are two problems with this API:

1. The product_query type is the wrong construct. product_query connotes an object that is intended to be used for "looking up" a data product. Instead, the provider is supplying a data product of a particular type, with a particular suffix, creator, and one or more associated layers.
2. The "provide_number" name may not be meaningful. It is not currently used in the lookup of data products.

[beojan]

I can take a look at this. Whatever we design for this should also be usable as the full product specification used to lookup products.

[beojan]

Here's a start: #501