Support for the --discover flag

[gaborbernat]

Resolves #2239

[codecov]

Codecov Report

Merging #2245 (14b82fe) into rewrite (cc04fe9) will increase coverage by 0.18%.
The diff coverage is 100.00%.

❗ Current head 14b82fe differs from pull request most recent head 0909788. Consider uploading reports for the commit 0909788 to get more accurate results

@@             Coverage Diff             @@
##           rewrite    #2245      +/-   ##
===========================================
+ Coverage    99.80%   99.98%   +0.18%     
===========================================
  Files          169      168       -1     
  Lines         9791     9694      -97     
  Branches      1001     1128     +127     
===========================================
- Hits          9772     9693      -79     
+ Misses           8        1       -7     
+ Partials        11        0      -11     

Flag	Coverage Δ
tests	99.98% <100.00%> (+0.18%)	⬆️

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

Impacted Files	Coverage Δ
src/tox/tox_env/python/virtual_env/api.py	100.00% <100.00%> (ø)
.../tox_env/python/virtual_env/test_virtualenv_api.py	100.00% <100.00%> (ø)
src/tox/tox_env/api.py	100.00% <0.00%> (ø)
src/tox/execute/request.py	100.00% <0.00%> (ø)
src/tox/util/pep517/backend.py	100.00% <0.00%> (ø)
src/tox/config/loader/str_convert.py	100.00% <0.00%> (ø)
...ecute/local_sub_process/read_via_thread_windows.py
.../execute/local_subprocess/test_local_subprocess.py	100.00% <0.00%> (+0.44%)	⬆️
src/tox/tox_env/python/api.py	100.00% <0.00%> (+0.69%)	⬆️
src/tox/util/spinner.py	100.00% <0.00%> (+2.15%)	⬆️
... and 1 more

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