Simplify specification of product-query fields

[knoepfel]

This enables a product_query specification like:

-    .input_family(product_query{.creator = "input"_id, .layer = "event"_id, .suffix = "id"_id})
+    .input_family(product_query{.creator = "input", .layer = "event", .suffix = "id"})

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #373      +/-   ##
==========================================
- Coverage   83.77%   83.69%   -0.09%     
==========================================
  Files         126      126              
  Lines        3291     3287       -4     
  Branches      583      582       -1     
==========================================
- Hits         2757     2751       -6     
  Misses        325      325              
- Partials      209      211       +2     

Flag	Coverage Δ
unittests	83.69% <100.00%> (-0.09%)	⬇️

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

Files with missing lines	Coverage Δ
phlex/core/product_query.hpp	81.81% <100.00%> (-4.85%)	⬇️
phlex/model/identifier.hpp	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[beojan]

The original design idea was that it should be very clear where you're creating an identifier vs a string, but that's somewhat fallen by the wayside after adding the std::string move constructor.

Can we back out the change to a single required class (while keeping the change to constructible_from) so we keep the clear error messages?

[knoepfel]

The original design idea was that it should be very clear where you're creating an identifier vs a string, but that's somewhat fallen by the wayside after adding the std::string move constructor.

Can we back out the change to a single required class (while keeping the change to constructible_from) so we keep the clear error messages?

Yes, I'll adjust it so we can maintain the clear error message