add --nodeps command line option

[janhn]

Kindly asking for a review of suggested changes to implement #410.

Tests have been run as suggested in the contributing guide, with and without the change to verify that no failures are being introduced.

=========================== short test summary info ============================
SKIP [1] tests/integration/test_package_int.py:38: flit is Python 3 only
SKIP [1] tests/unit/test_interpreters.py:30: non posix test
XFAIL unit/test_config.py::TestConfigTestEnv::()::test_regression_test_issue_706[envlist0]
  reproduce bug 706
XFAIL unit/test_config.py::TestCmdInvocation::()::test_colon_symbol_in_directory_name
  Upstream bug. See #203

============== 452 passed, 2 skipped, 2 xfailed in 100.44 seconds ==============

[codecov]

Codecov Report

Merging #1017 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1017      +/-   ##
==========================================
- Coverage   70.39%   70.38%   -0.02%     
==========================================
  Files          14       14              
  Lines        3486     3491       +5     
  Branches      465      466       +1     
==========================================
+ Hits         2454     2457       +3     
- Misses        971      974       +3     
+ Partials       61       60       -1

Impacted Files	Coverage Δ
src/tox/config.py	65.2% <100%> (ø)	⬆️
src/tox/venv.py	79.34% <100%> (-0.13%)	⬇️

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[gaborbernat]

Hello @janhn, thanks for the contribution. That being said I don't think this is the right direction to go, so I'll close this down now. Can you pick up the discussion and your use case under #410.