Complete type hinting of piptools.repositories package

[jdufresne]

Refs: #972

Changelog-friendly one-liner:

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 #1341 (b9d776f) into master (1d0c9b1) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1341      +/-   ##
==========================================
+ Coverage   99.62%   99.65%   +0.03%     
==========================================
  Files          33       33              
  Lines        2915     2923       +8     
  Branches      313      309       -4     
==========================================
+ Hits         2904     2913       +9     
  Misses          5        5              
+ Partials        6        5       -1     

Impacted Files	Coverage Δ
piptools/cache.py	100.00% <100.00%> (ø)
piptools/repositories/local.py	96.72% <100.00%> (+0.49%)	⬆️
piptools/repositories/pypi.py	97.18% <100.00%> (+0.06%)	⬆️
piptools/utils.py	100.00% <100.00%> (ø)
tests/test_utils.py	100.00% <100.00%> (+0.82%)	⬆️

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[atugushev]

Otherwise, LGTM! 👍🏼 So nice you've refactored lookup_table.