project: move imports from srcd.works to gopkg.in

[ajnavarro]

To be able to fix #261 we will move again to gopkg.in before v4 stable release.

[ajnavarro]

Merge src-d/go-billy#15 and src-d/go-git-fixtures#2 first.

[codecov]

Codecov Report

Merging #303 into master will increase coverage by 0.53%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #303      +/-   ##
==========================================
+ Coverage   77.19%   77.72%   +0.53%     
==========================================
  Files         117      108       -9     
  Lines        7937     7812     -125     
==========================================
- Hits         6127     6072      -55     
+ Misses       1159     1080      -79     
- Partials      651      660       +9

Impacted Files	Coverage Δ
plumbing/format/packfile/scanner.go	70.58% <ø> (ø)	✅
plumbing/object/treenoder.go	80.35% <ø> (ø)	✅
storage/filesystem/storage.go	87.5% <ø> (ø)	✅
plumbing/object/blob.go	73.58% <ø> (ø)	✅
submodule.go	65% <ø> (ø)	✅
plumbing/protocol/packp/sideband/muxer.go	91.3% <ø> (ø)	✅
plumbing/protocol/packp/advrefs.go	81.81% <ø> (ø)	✅
plumbing/object/commit_walker.go	87.09% <ø> (ø)	✅
storage/memory/storage.go	96.66% <ø> (ø)	✅
storage/filesystem/shallow.go	66.66% <ø> (ø)	✅
... and 78 more

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