Fix warnings about unused moduleRootPkg key

[fthomas]

This fixes these warnings:

[warn] there are 4 keys that are not used by any other settings/tasks:
[warn]
[warn] * benchmark / moduleRootPkg
[warn]   +- /home/runner/work/scala-steward/scala-steward/build.sbt:202
[warn] * mill-plugin / moduleRootPkg
[warn]   +- /home/runner/work/scala-steward/scala-steward/build.sbt:202
[warn] * root / moduleRootPkg
[warn]   +- /home/runner/work/scala-steward/scala-steward/build.sbt:305
[warn] * sbt-plugin / moduleRootPkg
[warn]   +- /home/runner/work/scala-steward/scala-steward/build.sbt:202
[warn]
[warn] note: a setting might still be used by a command; to exclude a key from this `lintUnused` check
[warn] either append it to `Global / excludeLintKeys` or call .withRank(KeyRanks.Invisible) on the key

[codecov]

Codecov Report

Merging #2016 (31dd3dc) into master (8e59be5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2016   +/-   ##
=======================================
  Coverage   78.59%   78.59%           
=======================================
  Files         131      131           
  Lines        2252     2252           
  Branches       52       52           
=======================================
  Hits         1770     1770           
  Misses        482      482           

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