Handle default gopath $HOME/go when env var $GOPATH is not set.

[Deleplace]

See golang/go#17262 about the implicit default gopath.

Also, this fixes the tests and "might" work better on Windows with multiple gopath entries (not thoroughly test by me yet).

[codecov]

Codecov Report

Merging #45 into master will decrease coverage by 0.77%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage   92.55%   91.77%   -0.78%     
==========================================
  Files          14       14              
  Lines         658      669      +11     
==========================================
+ Hits          609      614       +5     
- Misses         37       42       +5     
- Partials       12       13       +1

Impacted Files	Coverage Δ
gocomplete/pkgs.go	73.77% <50%> (-6.23%)	⬇️

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[Deleplace]

I'm not sure about the coverage warnings. Maybe due to half of the findGopath body never being executed on the codecov side?
I guess we can raise the coverage numbers by artificially tweaking the environment variables at runtime during tests (I have never done this, yet).
Tweaking the environment variables during tests may unfortunately jeopardize the t.Parallel() thing.

[posener]

I just keep the codecov warnings to know when a change is not covered, but I think that in this case we can accept it :-)

[posener]

Thanks for the fixes!