Fix "Plugin already registered" error with symlinks

[blueyed]

Fixes #4174.

[nicoddemus]

Great work, just need a changelog entry. 😁

[blueyed]

I will move this down to after the cache check.

added changelog

[codecov]

Codecov Report

Merging #4214 into master will increase coverage by 1.98%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4214      +/-   ##
==========================================
+ Coverage   93.82%   95.81%   +1.98%     
==========================================
  Files         109      109              
  Lines       24286    24296      +10     
  Branches     2392     2395       +3     
==========================================
+ Hits        22787    23279     +492     
+ Misses       1172      721     -451     
+ Partials      327      296      -31

Flag	Coverage Δ
#docs	28.5% <16.66%> (?)
#doctesting	28.5% <16.66%> (?)
#linting	28.5% <16.66%> (?)
#linux	95.59% <100%> (+3.32%)	⬆️
#nobyte	91.89% <100%> (?)
#numpy	92.9% <16.66%> (?)
#pexpect	41.26% <16.66%> (?)
#py27	93.95% <100%> (+2.92%)	⬆️
#py34	92.23% <100%> (+0.16%)	⬆️
#py35	92.24% <100%> (?)
#py36	93.9% <100%> (+1.84%)	⬆️
#py37	92.26% <100%> (?)
#trial	92.9% <16.66%> (?)
#windows	94.03% <16.66%> (+3%)	⬆️
#xdist	93.78% <100%> (?)

Impacted Files	Coverage Δ
src/_pytest/config/__init__.py	94.93% <100%> (+0.34%)	⬆️
testing/test_conftest.py	99.6% <100%> (ø)	⬆️
testing/code/test_code.py	94.11% <0%> (-1.34%)	⬇️
testing/code/test_source.py	97.02% <0%> (ø)	⬆️
testing/python/fixture.py	99.24% <0%> (+0.1%)	⬆️
src/_pytest/assertion/rewrite.py	95.68% <0%> (+0.16%)	⬆️
testing/deprecated_test.py	100% <0%> (+0.77%)	⬆️
testing/test_parseopt.py	94.65% <0%> (+0.82%)	⬆️
src/_pytest/pytester.py	87.3% <0%> (+0.86%)	⬆️
... and 17 more

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[nicoddemus]

Thanks! 👍