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   85.36%   85.22%   -0.15%     
==========================================
  Files         122      122              
  Lines        2433     2430       -3     
  Branches      389      388       -1     
==========================================
- Hits         2077     2071       -6     
+ Misses        233      232       -1     
- Partials      123      127       +4     

Flag	Coverage Δ
unittests	85.22% <100.00%> (-0.15%)	⬇️

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	90.00% <100.00%> (-2.86%)	⬇️
phlex/model/identifier.cpp	94.44% <100.00%> (+0.32%)	⬆️
phlex/model/identifier.hpp	100.00% <ø> (ø)

... and 3 files with indirect coverage changes

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