cloud/v1: Aggregation in a dedicated pkg

[codebien]

It splits the current output in a subpackage v1, it moves there the specific parts for version 1 and that are not expected to be used from version 2.
It uses part of the work done in #3041, and it is a foundation for adding, on top of it, the versioning config option (#3041) and the expv2 package (#2963).

The proposed architecture should resolve the requested change in #2963 (comment).

I think you should review the code going through the commits one by one to check the evolution, otherwise, the final code could be too chaotic.

I think the test coverage is not high enough for the new output/cloud/output.go to have good confidence. If the architecture is confirmed I will add a bunch of them in a dedicated PR (we can keep this open till it is approved).

[codecov-commenter]

Codecov Report

Merging #3063 (f90d741) into master (c00bfbb) will decrease coverage by 0.04%.
The diff coverage is 71.34%.

❗ Current head f90d741 differs from pull request most recent head 10ecd76. Consider uploading reports for the commit 10ecd76 to get more accurate results

@@            Coverage Diff             @@
##           master    #3063      +/-   ##
==========================================
- Coverage   76.72%   76.69%   -0.04%     
==========================================
  Files         229      230       +1     
  Lines       17110    17173      +63     
==========================================
+ Hits        13127    13170      +43     
- Misses       3140     3153      +13     
- Partials      843      850       +7     

Flag	Coverage Δ
ubuntu	76.69% <71.34%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
output/cloud/v1/cloud_easyjson.go	35.55% <ø> (ø)
output/cloud/v1/data.go	91.24% <ø> (ø)
output/cloud/v1/metrics_client.go	80.00% <ø> (ø)
cmd/cloud.go	70.40% <57.14%> (-1.18%)	⬇️
output/cloud/v1/output.go	67.81% <67.81%> (ø)
cloudapi/api.go	54.45% <71.42%> (+2.37%)	⬆️
output/cloud/output.go	66.90% <72.50%> (-2.71%)	⬇️
js/bundle.go	85.23% <100.00%> (+0.66%)	⬆️
lib/testutils/logrus_hook.go	100.00% <100.00%> (ø)
metrics/registry.go	95.34% <100.00%> (+3.85%)	⬆️
... and 1 more

... and 5 files with indirect coverage changes

[codebien]

Rebased to resolve the conflicts, green now

[olegbespalov]

LGTM, with a few minor non-blocking comments 👍