build: modular build structure

[alandefreitas]

This is a version of https://github.com/boostorg/url/pull/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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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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