Add package_root support

[gaborbernat]

Resolves #1838.

[codecov]

Codecov Report

Merging #1891 (f4f2fe1) into rewrite (716e79d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           rewrite    #1891   +/-   ##
========================================
  Coverage    99.81%   99.81%           
========================================
  Files          135      135           
  Lines         7603     7622   +19     
  Branches       762      764    +2     
========================================
+ Hits          7589     7608   +19     
  Misses           3        3           
  Partials        11       11           

Flag	Coverage Δ
tests	99.81% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/tox/tox_env/package.py	100.00% <100.00%> (ø)
src/tox/tox_env/python/package.py	100.00% <100.00%> (ø)
src/tox/tox_env/python/virtual_env/package/api.py	100.00% <100.00%> (ø)
tests/tox_env/python/virtual_env/test_package.py	100.00% <100.00%> (ø)

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