Decoupled crane logic from crane command line code

[mschwrz]

• Added commands package
• Added api package
• Extracted logic from commands into api package

This should resolve #407

[codecov-io]

Codecov Report

Merging #476 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #476   +/-   ##
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  Coverage   62.84%   62.84%           
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  Files          76       76           
  Lines        3722     3722           
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  Hits         2339     2339           
  Misses       1074     1074           
  Partials      309      309

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[jonjohnsonjr]

Oof, I wish I had seen your comment yesterday. I had been working on this recently, and it's slightly different from what you had here. I've rebased that and pushed it here: #478

Instead of doing a direct CLI -> package translation, I tried to tease apart what crane does into a set of functions that is usable without having to spend too much time learning the abstractions in this library, e.g.:

$ crane pull == crane.Save(crane.Pull(...))

$ crane export == crane.Export(crane.Pull(...))

$ crane push == crane.Push(crane.Load(...))

because this would allow someone who wanted to export the filesystem of a tarball image to easily discover they could do so via:

crane.Export(crane.Load(...))

Apologies for not responding -- I don't mean to waste your time. I'd love your feedback on my PR if you're interested 😄

[mschwrz]

No Problem, I just could have waited for your response before starting to work on it 😄 The time isn't wasted at all, I'm currently learning go and was searching for some practical examples.

I like your approach in solving this issue. I also considered coding it like that, but discarded it.
I discarded it because I saw that ko kept its ko/cmd/ko directory very clean by only putting the main-logic into it and placed the commands themselves into the pkg/commands directory.
With this approach, ko nicely decoupled the commands from the main logic.

Maybe we can try something like this:

• The "Business Logic" of crane goes into pkg/crane, so the user can still use crane.foo(), without having to rename pkg/crane/api to crane on importing it, as you designed it in your PR
• The command logic goes into pkg/crane/commands, to keep the cmd/crane directory clean

[jonjohnsonjr]

I think what we have is pretty close to this now, going to close this PR but feel free to reopen if I missed something.