Fix collection of symlinked files

[blueyed]

Fixes #4325.
Fixes #4321.

TODO:

• Regression: item.nodeid starts with '::' when symlinks are involved #4321 (comment)

[codecov]

Codecov Report

Merging #4337 into master will decrease coverage by <.01%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master    #4337      +/-   ##
==========================================
- Coverage   95.85%   95.84%   -0.01%     
==========================================
  Files         111      111              
  Lines       24910    25071     +161     
  Branches     2431     2448      +17     
==========================================
+ Hits        23877    24029     +152     
- Misses        736      745       +9     
  Partials      297      297

Flag	Coverage Δ
#docs	28.88% <37.03%> (ø)	⬆️
#doctesting	28.88% <37.03%> (ø)	⬆️
#linting	28.88% <37.03%> (ø)	⬆️
#linux	95.61% <96.29%> (-0.03%)	⬇️
#nobyte	91.83% <96.29%> (-0.02%)	⬇️
#numpy	92.38% <77.77%> (-0.63%)	⬇️
#pexpect	41.64% <44.44%> (+0.01%)	⬆️
#py27	94.01% <96.29%> (-0.02%)	⬇️
#py34	92.14% <96.29%> (+0.04%)	⬆️
#py35	92.15% <96.29%> (+0.05%)	⬆️
#py36	93.92% <96.29%> (+0.02%)	⬆️
#py37	92.19% <96.29%> (+0.05%)	⬆️
#trial	92.38% <77.77%> (-0.63%)	⬇️
#windows	93.97% <77.77%> (+0.01%)	⬆️
#xdist	93.73% <96.29%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
src/_pytest/config/__init__.py	95.02% <100%> (ø)	⬆️
src/_pytest/main.py	96.88% <100%> (-0.01%)	⬇️
testing/test_collection.py	99.78% <100%> (ø)	⬆️
src/_pytest/config/findpaths.py	94.11% <92.85%> (-0.73%)	⬇️
testing/test_modimport.py	69.23% <0%> (-12.59%)	⬇️
testing/test_argcomplete.py	66.66% <0%> (-1.45%)	⬇️
src/_pytest/capture.py	93.42% <0%> (-0.46%)	⬇️
src/_pytest/python.py	95.27% <0%> (-0.25%)	⬇️
testing/test_session.py	96.5% <0%> (ø)	⬆️
testing/test_nodes.py	100% <0%> (ø)	⬆️
... and 1 more

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[Lekensteyn]

#4321 is not completely solved by this patch, see #4321 (comment) for a reproducer. The leading colons (::) are gone, but the symlink name is unexpectedly included in the nodeid.

[blueyed]

@nicoddemus
Please merge if it is OK with you.

[nicoddemus]

Awesome, thanks!