Port pip requirements.txt parser

[gaborbernat]

The plan is to move out the requirements file parser to a separate library and allow pip to vendor it.

Signed-off-by: Bernát Gábor [email protected]

[codecov]

Codecov Report

Merging #2009 (8e83086) into rewrite (1a50180) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           rewrite    #2009    +/-   ##
=========================================
  Coverage    99.83%   99.83%            
=========================================
  Files          147      151     +4     
  Lines         8286     8455   +169     
  Branches       825      876    +51     
=========================================
+ Hits          8272     8441   +169     
  Misses           3        3            
  Partials        11       11            

Flag	Coverage Δ
tests	99.83% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/tox/config/loader/stringify.py	100.00% <100.00%> (ø)
src/tox/tox_env/package.py	100.00% <100.00%> (ø)
src/tox/tox_env/python/pip/pip_install.py	100.00% <100.00%> (ø)
src/tox/tox_env/python/pip/req/args.py	100.00% <100.00%> (ø)
src/tox/tox_env/python/pip/req/file.py	100.00% <100.00%> (ø)
src/tox/tox_env/python/pip/req/util.py	100.00% <100.00%> (ø)
src/tox/tox_env/python/pip/req_file.py	100.00% <100.00%> (ø)
tests/tox_env/python/pip/req/test_file.py	100.00% <100.00%> (ø)
tests/tox_env/python/pip/test_pip_install.py	100.00% <100.00%> (ø)
tests/tox_env/python/pip/test_req_file.py	100.00% <100.00%> (ø)
... and 6 more

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