Rename DependencyService to RepoCacheAlg and store sbt versions

[fthomas]

Storing the sbt versions will be useful later to determine which repos
use an outdated sbt version.

[codecov]

Codecov Report

Merging #692 into master will decrease coverage by <.01%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master     #692      +/-   ##
==========================================
- Coverage   61.03%   61.02%   -0.01%     
==========================================
  Files          78       79       +1     
  Lines        1065     1070       +5     
  Branches       22       24       +2     
==========================================
+ Hits          650      653       +3     
- Misses        415      417       +2

Impacted Files	Coverage Δ
.../scala/org/scalasteward/core/update/splitter.scala	0% <ø> (ø)	⬆️
...la/org/scalasteward/core/application/Context.scala	0% <ø> (ø)	⬆️
.../main/scala/org/scalasteward/core/sbt/SbtAlg.scala	71.18% <ø> (ø)	⬆️
...g/scalasteward/core/repocache/json/RepoStore.scala	0% <ø> (ø)
...la/org/scalasteward/core/repocache/RepoCache.scala	0% <ø> (ø)
...scalasteward/core/sbt/data/ArtificialProject.scala	100% <ø> (ø)	⬆️
.../main/scala/org/scalasteward/core/sbt/parser.scala	100% <ø> (ø)	⬆️
...teward/core/nurture/json/JsonPullRequestRepo.scala	0% <ø> (ø)	⬆️
...scala/org/scalasteward/core/model/Dependency.scala	75% <ø> (ø)
...a/org/scalasteward/core/update/UpdateService.scala	0% <0%> (ø)	⬆️
... and 8 more

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[fthomas]

The second commit also caches RepoConfigs so that we can exclude ignored updates in the update check in UpdateService.