Do not raise UsageError when "pytest_plugins" is a module

[nicoddemus]

Fix #3899

[codecov]

Codecov Report

Merging #4728 into master will decrease coverage by 52.74%.
The diff coverage is 16.66%.

@@             Coverage Diff             @@
##           master    #4728       +/-   ##
===========================================
- Coverage   95.68%   42.94%   -52.75%     
===========================================
  Files         113       93       -20     
  Lines       24963    20875     -4088     
  Branches     2478     2368      -110     
===========================================
- Hits        23886     8964    -14922     
- Misses        762    11309    +10547     
- Partials      315      602      +287

Flag	Coverage Δ
#docs	29.59% <16.66%> (-0.01%)	⬇️
#doctesting	29.59% <16.66%> (-0.01%)	⬇️
#linting	29.59% <16.66%> (-0.01%)	⬇️
#linux	42.94% <16.66%> (-52.57%)	⬇️
#nobyte	?
#numpy	41.2% <16.66%> (-51.92%)	⬇️
#pexpect	41.2% <16.66%> (-0.91%)	⬇️
#py27	41.2% <16.66%> (-52.48%)	⬇️
#py34	?
#py35	?
#py36	?
#py37	?
#trial	41.2% <16.66%> (-51.92%)	⬇️
#windows	?
#xdist	?

Impacted Files	Coverage Δ
testing/acceptance_test.py	0% <0%> (-97.85%)	⬇️
src/_pytest/config/__init__.py	69.55% <50%> (-24.13%)	⬇️
testing/test_pastebin.py	0% <0%> (-100%)	⬇️
testing/test_resultlog.py	0% <0%> (-100%)	⬇️
testing/test_pluginmanager.py	0% <0%> (-100%)	⬇️
testing/test_entry_points.py	0% <0%> (-100%)	⬇️
testing/test_doctest.py	0% <0%> (-100%)	⬇️
testing/test_runner_xunit.py	0% <0%> (-100%)	⬇️
testing/test_pathlib.py	0% <0%> (-100%)	⬇️
testing/test_nodes.py	0% <0%> (-100%)	⬇️
... and 94 more

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[RonnyPfannschmidt]

well done, waiting for ci