Fix 'Package has no len()' error during collection

[nicoddemus]

Fix #3749

[coveralls]

Coverage increased (+0.02%) to 93.809% when pulling 41f6ea1 on nicoddemus:package-len-error-3749 into f6eb39d on pytest-dev:master.

[codecov]

Codecov Report

Merging #4010 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #4010      +/-   ##
=========================================
+ Coverage   94.45%   94.5%   +0.04%     
=========================================
  Files         109     109              
  Lines       23778   23782       +4     
  Branches     2357    2357              
=========================================
+ Hits        22460   22474      +14     
+ Misses       1006    1000       -6     
+ Partials      312     308       -4

Flag	Coverage Δ
#doctesting	29.37% <33.33%> (+0.06%)	⬆️
#linux	94.35% <100%> (+0.04%)	⬆️
#nobyte	0% <ø> (ø)	⬆️
#numpy	28.28% <33.33%> (+0.06%)	⬆️
#pexpect	0% <ø> (ø)	⬆️
#py27	92.62% <100%> (+0.04%)	⬆️
#py34	92.11% <100%> (+0.03%)	⬆️
#py35	92.12% <100%> (+0.03%)	⬆️
#py36	92.69% <100%> (+0.04%)	⬆️
#py37	92.32% <100%> (+0.03%)	⬆️
#trial	31.31% <33.33%> (+0.06%)	⬆️
#windows	93.8% <100%> (ø)	⬆️
#xdist	18.58% <0%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
testing/python/collect.py	99.29% <100%> (ø)	⬆️
src/_pytest/main.py	96.19% <100%> (ø)	⬆️
src/_pytest/terminal.py	91.6% <0%> (+1.74%)	⬆️

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