Put all global sbt plugins in scala-steward.sbt

[fthomas]

The reason for this is to better distinguish what scala-steward
installed and everything else.

[codecov]

Codecov Report

Merging #581 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #581      +/-   ##
==========================================
+ Coverage   59.04%   59.09%   +0.04%     
==========================================
  Files          67       67              
  Lines         857      858       +1     
  Branches       38       40       +2     
==========================================
+ Hits          506      507       +1     
  Misses        351      351

Impacted Files	Coverage Δ
.../main/scala/org/scalasteward/core/sbt/SbtAlg.scala	47.5% <100%> (-1.29%)	⬇️
...main/scala/org/scalasteward/core/sbt/package.scala	66.66% <100%> (+6.66%)	⬆️

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