Show default index url provided by pip

[atugushev]

Closes #705.

Changelog-friendly one-liner: Show default index url provided by pip

Contributor checklist

• Provided the tests for the changes.
• Requested a review from another contributor.
• 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 #735 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #735      +/-   ##
==========================================
+ Coverage   86.25%   86.33%   +0.07%     
==========================================
  Files          33       34       +1     
  Lines        2117     2129      +12     
  Branches      303      303              
==========================================
+ Hits         1826     1838      +12     
  Misses        226      226              
  Partials       65       65

Impacted Files	Coverage Δ
tests/test_repositories.py	100% <100%> (ø)	⬆️
tests/test_cli_compile.py	100% <100%> (ø)	⬆️
piptools/scripts/compile.py	86.92% <100%> (-0.94%)	⬇️
piptools/pip.py	100% <100%> (ø)
tests/test_repository_local.py	100% <100%> (ø)	⬆️
tests/test_top_level_editable.py	100% <100%> (ø)	⬆️
tests/test_repository_pypi.py	100% <100%> (ø)	⬆️

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[atugushev]

I've resolved conflicts, rebased and squashed commits. I'll merge this as soon as tests are passed.

@blueyed as always, appreciate your feedback!

[atugushev]

pip-tools v3.5.0 is released