Added possibility to fetch as 'git clone'

[shmel1k]

Hi. Answering to this #316 issue. I added possibility to clone repo using 'gb vendor fetch -clone' in order to support this task. It is not a git submodule at all, but allows to manage repository into vendor folder.

[codecov-io]

Codecov Report

Merging #703 into master will decrease coverage by 0.4%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #703      +/-   ##
==========================================
- Coverage   53.32%   52.91%   -0.41%     
==========================================
  Files          40       42       +2     
  Lines        3036     3152     +116     
==========================================
+ Hits         1619     1668      +49     
- Misses       1241     1341     +100     
+ Partials      176      143      -33

Impacted Files	Coverage Δ
internal/vendor/manifest.go	32.81% <ø> (ø)	⬆️
internal/fileutils/fileutils.go	19.6% <66.66%> (+1.6%)	⬆️
depfile.go	8% <0%> (-24.07%)	⬇️
internal/depfile/depfile.go	87.65% <0%> (ø)
cmd/gb/build.go	4.87% <0%> (ø)
build.go	85.33% <0%> (+5.2%)	⬆️

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