Refactor package structure

[kakkoyun]

This PR changes the package structure to make it more maintainable. This need has risen after several backends merged.

Fixes several issues.
Improves automated tests.
Adopts stream-based IO processing.

Proposed Changes

• Log actual compression ratio
  • Fixes Feature request: log gzip compression ratio #72.
  • Log actually written bytes.
  • Log final artefact size in bytes.
  • Log ratio.
• Refactor packages to make it more readable, testable and maintainable after the introduction of multiple storage backends.
  • Add missing documentation.
  • Introduce the archive package.
  • Introduce the storage package.
  • Introduce stream-based io ops.
• Update contributing docs.
• Add more markdown static change.
• AWS Multipart uploads
  • Fixes Cannot upload files larger than 5GB to AWS S3 in a single PUT operation #55.
  • Fixes When can you provide a s3 fixed-size patch upload method? #88.
• Update README, add backends.
  • Fixes Add Supported storage backend section #79.
• Add timeout option.
  • Fixes Add timeout option to cancel outgoing request after the given time #74.
• Improve tests.
  • Fixes Improve quality of automated tests #45.
• Handle nested paths.
  • Fixes Cache cannot rebuid properly when nested paths specified in mount #87.
• Make local and remote root configurable.
  • Fixes Customize cache key with path #97.

Checklist

• Read the CONTRIBUTING document.
• Read the CODE OF CONDUCT document.
• Add tests to cover changes.
• Ensure your code follows the code style of this project.
• Ensure CI and all other PR checks are green OR
  • Code compiles correctly.
  • Created tests which fail without the change (if possible).
  • All new and existing tests passed.
• Add your changes to Unreleased section of CHANGELOG.
• Improve and update the README (if necessary).
• Ensure documentation is up-to-date. The same file will be updated in plugin index when your PR is accepted, so it will be available for end-users at http://plugins.drone.io.

[kakkoyun]

Update for the eyes of accidental reviewer:

This branch has gotten out of control. however please bear with me. It introduces a major overhaul on the structure and flow. This changes had been urged after the addition of several storage backends to the project. Roughly %70 of the work is done. It mostly misses tests, it'd be ready after necessary tests are added.

All the issue will that would be fixed when this got merged is listed in the description.

[aweris]

I think this PR too big to merge. I think you should split this PR to a couple of smaller PR's

[aweris]

Excellent work, well done!

Added a few recommendations about general commit structure and changesets.

Since it's a huge PR, I think you should replan this PR and split it smaller PR's and reorganize your commits. Each commit should explicit and contain a single change.

[aweris]

$(GOPATH)/bin used a couple of times already. Maybe it could be a good idea to move this to GOBIN=$(GOPATH)/bin

[kakkoyun]

I will address this in another PR

[kakkoyun]

@aweris Thanks for the great review and feedback. After I'm done, I will separate it into more digestible chunks a thorough review.

[kakkoyun]

I have cleaned the commit history. This is now ready to merge.
Thanks, @aweris and silent others for the comments.

[kakkoyun]

I'm gonna go ahead and merge this. I'll address if someone reviews it (unlikely :)) and find an issue.