feat(vendir): embed vendir into myks

[kbudde]

Highly experimental and open for discussion: This PR includes vendir as part of myks.
myks vendir is used as part of the sync process.

• Embedding vendir will prevent all kind of issues with too old or too new version of vendir
• We can embed versions before release, e.g. as needed for fix(sync): allow local paths in vendir config #191

[kbudde]

This line is important ;)

[codecov]

Codecov Report

Attention: 34 lines in your changes are missing coverage. Please review.

Comparison is base (053e89f) 47.54% compared to head (09c3a11) 46.93%.
Report is 1 commits behind head on dev.

Files	Patch %	Lines
cmd/vendir/vendir.go	0.00%	33 Missing ⚠️
cmd/root.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #199      +/-   ##
==========================================
- Coverage   47.54%   46.93%   -0.62%     
==========================================
  Files          26       27       +1     
  Lines        2509     2544      +35     
==========================================
+ Hits         1193     1194       +1     
- Misses       1131     1165      +34     
  Partials      185      185              

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[Zebradil]

That is really neat, I like the idea.

I thought to embed vendir and ytt from the beginning of the project, but it was more difficult than I thought. Apart from that, those tools' maintainers suggested using binaries rather than libraries to avoid breaking changes. But it's convenient to have all the required tools shipped with myks.

I see the benefit of using vendir as a subcommand in that it provides the full functionality of vendir to the end user when it's needed. In the single place where the vendir command is used in our code, we can also invoke vendir directly without creating a sub-process. But that can come later.

I don't see any issues with the current approach so far, we should just try it for some time.