Do not search for best_match for -c constraints that are not (yet) requirements

[clslgrnc]

Fixes #1178.

In my organization we use a mono-repository with a huge constraints file for security purposes (via the -c option).
Additionally we have a few internal libs with a lot of versions.

As a result pip-compile is extremely slow as it tries to find a match for all constraints in our constraints file, at each round.
This is not necessary until these constraints become requirements.

The debug line should probably be changed, as an example we have:

found candidate urllib3>1.25.7 (constraint was >1.25.7)

Maybe I should change it to:

unused constraint urllib3>1.25.7

or something like that.

Changelog-friendly one-liner: Do not find_best_match for constraints (as opposed to requirements)

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 #1175 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1175   +/-   ##
=======================================
  Coverage   99.50%   99.50%           
=======================================
  Files          36       36           
  Lines        2839     2841    +2     
  Branches      333      334    +1     
=======================================
+ Hits         2825     2827    +2     
  Misses          8        8           
  Partials        6        6           

Impacted Files	Coverage Δ
tests/test_resolver.py	100.00% <ø> (ø)
piptools/resolver.py	99.37% <100.00%> (+<0.01%)	⬆️

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[atugushev]

Thanks for the improvement! Could you add a test which would cover that case?

[clslgrnc]

Added test fails without the patch

[atugushev]

Awesome! Thanks.

[atugushev]

pip-tools v5.3.0 is released.