Start plugin interface documentation, add installer

[gaborbernat]

• Add documentation for the plugin interface
• Introduce the installer abstraction
• Rework how we handle tox deps section, requirement and constraint files
• Support for escaping comments in tox.ini configs

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

[codecov]

Codecov Report

Merging #1991 (089c9b3) into rewrite (54e6310) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           rewrite    #1991    +/-   ##
=========================================
  Coverage    99.82%   99.83%            
=========================================
  Files          146      147     +1     
  Lines         7948     8276   +328     
  Branches       792      823    +31     
=========================================
+ Hits          7934     8262   +328     
  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/cli/parser.py	100.00% <ø> (ø)
src/tox/config/sets.py	100.00% <ø> (ø)
src/tox/execute/local_sub_process/__init__.py	95.67% <ø> (-0.03%)	⬇️
.../config/loader/ini/replace/test_replace_posargs.py	100.00% <ø> (ø)
tests/config/loader/ini/test_ini_loader.py	100.00% <ø> (ø)
src/tox/config/cli/parse.py	100.00% <100.00%> (ø)
src/tox/config/loader/api.py	100.00% <100.00%> (ø)
src/tox/config/loader/convert.py	100.00% <100.00%> (ø)
src/tox/config/loader/ini/__init__.py	100.00% <100.00%> (ø)
src/tox/config/loader/stringify.py	100.00% <100.00%> (ø)
... and 58 more

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