Hide afero behind fsext package

[mstoykov]

This is mostly so that no other part by fsext knows anything about afero.

In the future we can replace the actual implementation.

Updates #1079

[codecov-commenter]

Codecov Report

Merging #3025 (5ecacfc) into master (d946fdb) will decrease coverage by 0.02%.
The diff coverage is 76.47%.

❗ Current head 5ecacfc differs from pull request most recent head 8b7f619. Consider uploading reports for the commit 8b7f619 to get more accurate results

@@            Coverage Diff             @@
##           master    #3025      +/-   ##
==========================================
- Coverage   77.01%   77.00%   -0.02%     
==========================================
  Files         228      229       +1     
  Lines       17063    17071       +8     
==========================================
+ Hits        13141    13145       +4     
- Misses       3079     3084       +5     
+ Partials      843      842       -1     

Flag	Coverage Δ
ubuntu	77.00% <76.47%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/convert.go	68.67% <0.00%> (ø)
cmd/run.go	72.76% <ø> (ø)
cmd/state/state.go	5.71% <0.00%> (ø)
cmd/test_load.go	79.71% <ø> (ø)
js/bundle.go	84.57% <ø> (ø)
js/common/initenv.go	66.66% <0.00%> (ø)
js/runner.go	84.38% <ø> (-0.47%)	⬇️
log/file.go	79.66% <ø> (ø)
cmd/config.go	86.79% <50.00%> (ø)
js/initcontext.go	80.00% <50.00%> (ø)
... and 6 more

... and 3 files with indirect coverage changes

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[codebien]

I'm for not including it in the current release and merging it at the beginning of the next cycle.

[mstoykov]

I'm for not including it in the current release and merging it at the beginning of the next cycle.

This is why it is milestoned for 45 ;)