build: modular build structure

[alandefreitas]

This is a version of #850 that's been rebased and squashed.

[cppalliance-bot]

An automated preview of the documentation is available at https://860.url.prtest2.cppalliance.org/libs/url/doc/html/index.html

[cppalliance-bot]

Antora version: an automated preview of the documentation is available at https://860.urlantora.prtest2.cppalliance.org/site/index.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.21%. Comparing base (dcfe9df) to head (dc611e8).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #860   +/-   ##
========================================
  Coverage    99.21%   99.21%           
========================================
  Files          157      157           
  Lines         8421     8421           
========================================
  Hits          8355     8355           
  Misses          66       66           

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[cppalliance-bot]

GCOVR code coverage report https://860.url.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://860.url.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff https://860.url.prtest2.cppalliance.org/gcovr/coverage_diff.txt

[cppalliance-bot]

Antora version: an automated preview of the documentation is available at https://860.urlantora.prtest2.cppalliance.org/site/index.html

[cppalliance-bot]

An automated preview of the documentation is available at https://860.url.prtest2.cppalliance.org/libs/url/doc/html/index.html

[cppalliance-bot]

GCOVR code coverage report https://860.url.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://860.url.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff https://860.url.prtest2.cppalliance.org/gcovr/coverage_diff.txt