changes to loader.clj to make kondo/codeclimate more happy

[gkspranger]

• dropped non-used refs
• added some doc strings
• variablized regex related to "all" commands and observers
• used new regex vars to remove (last) logic in (yetibot-all-namespaces) << uncle bob would be proud
• small style changes in (try) << make kondo happy
• using (doseq) vs (dorun) for side effects << make codeclimate happy
• added comments and (comment)

[codecov[bot]]

Codecov Report

Merging #160 (44f8862) into master (465de69) will increase coverage by 0.12%. The diff coverage is 96.00%.

@@            Coverage Diff             @@
##           master     #160      +/-   ##
==========================================
+ Coverage   45.84%   45.96%   +0.12%     
==========================================
  Files          94       94              
  Lines        4101     4112      +11     
  Branches      196      197       +1     
==========================================
+ Hits         1880     1890      +10     
  Misses       2025     2025              
- Partials      196      197       +1     

Impacted Files	Coverage Δ
src/yetibot/core/loader.clj	71.62% <96.00%> (+3.36%)	:arrow_up:

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