Separate internal and plugin output sections

[medikoo]

Goal is to show sections as introduced by plugins after sections set in the core. For that we need to use different section collections.

This PR proposes so serverless.addServiceOutputSection is dedicated just for use within plugins, and internally we add sections directly into serviceOutputs collection.

[codecov]

Codecov Report

Merging #10184 (ef65159) into master (d202af1) will decrease coverage by 0.04%.
The diff coverage is 76.47%.

❗ Current head ef65159 differs from pull request most recent head 968010b. Consider uploading reports for the commit 968010b to get more accurate results

@@            Coverage Diff             @@
##           master   #10184      +/-   ##
==========================================
- Coverage   85.38%   85.34%   -0.05%     
==========================================
  Files         339      339              
  Lines       13819    13822       +3     
==========================================
- Hits        11800    11797       -3     
- Misses       2019     2025       +6     

Impacted Files	Coverage Δ
lib/Serverless.js	66.96% <33.33%> (-2.51%)	⬇️
lib/plugins/aws/info/display.js	75.67% <80.00%> (ø)
lib/plugins/aws/deploy/index.js	98.64% <100.00%> (+0.01%)	⬆️
lib/plugins/aws/info/index.js	97.22% <100.00%> (+0.07%)	⬆️

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[pgrzesik]

Looks great 👍