Move provider API away from misusing product_query for outputs

[beojan]

Replaces #501

[codecov]

Codecov Report

❌ Patch coverage is 96.49123% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
phlex/core/product_query.cpp	71.42%	1 Missing and 1 partial ⚠️

@@            Coverage Diff             @@
##             main     #602      +/-   ##
==========================================
+ Coverage   82.16%   82.25%   +0.09%     
==========================================
  Files         157      157              
  Lines        5730     5760      +30     
  Branches      646      649       +3     
==========================================
+ Hits         4708     4738      +30     
+ Misses        808      807       -1     
- Partials      214      215       +1     

Flag	Coverage Δ
scripts	76.12% <ø> (ø)
unittests	82.25% <96.49%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
form/form_source.cpp	0.00% <ø> (ø)
phlex/core/edge_maker.cpp	77.27% <100.00%> (+1.08%)	⬆️
phlex/core/product_query.hpp	86.66% <ø> (ø)
phlex/core/provider_node.cpp	100.00% <100.00%> (+4.54%)	⬆️
phlex/core/provider_node.hpp	100.00% <ø> (ø)
phlex/core/registration_api.hpp	100.00% <100.00%> (ø)
plugins/python/src/modulewrap.cpp	80.34% <100.00%> (+0.34%)	⬆️
phlex/core/product_query.cpp	65.30% <71.42%> (+1.02%)	⬆️

... and 4 files with indirect coverage changes

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[knoepfel]

@beojan, this is looking quite good. I noticed that you've chosen the explicit experimental::algorithm_name::create("input") instead of just "input". I think we should be able to support both patterns. Probably involves an output_product(...) template that is constrained on std::constructible_from<> or std::is_convertible<> or ...?

My only other comment was in cleaning up the match(...) implementation, which you already addressed while I was assembling this review.

[beojan]

As it happens, we already have an implicit constructor from string literals to algorithm_name so we didn't need to explicitly call create.