[WIP] Add support for file based --extra-index-url to pip-sync

[altendky]

Any --extra-index-url that are added inside the requirements.txt file are ignored. This change makes sure they are honoured.

This is a catchup for #674

Changelog-friendly one-liner: Added support for --extra-index-urls options inside requirements.txt files

Contributor checklist

• Provided the tests for the changes.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[codecov]

Codecov Report

Merging #951 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #951      +/-   ##
==========================================
+ Coverage    99.1%   99.11%   +0.01%     
==========================================
  Files          35       35              
  Lines        2336     2377      +41     
  Branches      301      307       +6     
==========================================
+ Hits         2315     2356      +41     
  Misses         13       13              
  Partials        8        8

Impacted Files	Coverage Δ
piptools/scripts/sync.py	100% <100%> (ø)	⬆️
tests/test_cli_sync.py	100% <100%> (ø)	⬆️
tests/test_cli_compile.py	100% <100%> (ø)	⬆️
tests/utils.py	100% <100%> (ø)	⬆️
tests/conftest.py	98.23% <100%> (+0.38%)	⬆️

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[atugushev]

Hello @altendky,

Thanks for working on this! FTR, It's also implemented in #824 as a comprehensive solution with support the other options, but a little bit stalled.

[atugushev]

Sorry, accidentally closed PR :(

[altendky]

Guess I should have been more careful about what PR to update. :[ Probably ought to just close these if #824 does it all I guess.

[atugushev]

@altendky

I've rebased #824 and refactored it, could you review/test it?

[atugushev]

Superseded by #824.