Support Scalafix migrations that target build files

[fthomas]

What has been done in this PR?

This PR extends the Scalafix support to run migrations with the Scalafix CLI in addition to sbt-scalafix. Migrations that are executed with the CLI target *.sbt and project/*.scala files in the build root directories. So these build migrations can change build files while regular sources migrations can change source files.

How?

To distinguish between sources and build migrations a new optional target field is added to ScalafixMigration. The default value is sources if this field is not specified. Scala Steward differentiate these two targets in SbtAlg and either calls the proper sbt command or the scalafix CLI. A new ScalafixCli class is added that takes care of running migrations with the CLI.

Want to see it in action?

This PR also adds @eed3si9n's Sbt0_13BuildSyntax for unified slash syntax to our list of Scalafix migrations:

scala-steward/modules/core/src/main/resources/scalafix-migrations.conf

Lines 92 to 100 in 2018290

	{
	groupId: "org.scala-sbt",
	artifactIds: ["sbt"],
	newVersion: "1.5.0",
	rewriteRules: ["https://gist.githubusercontent.com/eed3si9n/57e83f5330592d968ce49f0d5030d4d5/raw/7f576f16a90e432baa49911c9a66204c354947bb/Sbt0_13BuildSyntax.scala"],
	doc: "https://eed3si9n.com/syntactic-scalafix-rule-for-unified-slash-syntax",
	authors: ["Eugene Yokota (eed3si9n) <eed3si9n@gmail.com>"]
	target: "build"
	},

which is used when sbt is bumped over 1.5.0. An example PR where the above migration was used and contains changes to build.sbt from Scalafix is:

• Update sbt to 1.5.5 in slash-syntax-2 test-repo-2#55

The above PR also demonstrate another new feature: Co-authored-by: attribution of the Scalafix rule authors (#1914). @eed3si9n I added your name to the authors field above to test that feature but I'll only keep your name there if you're fine with this attribution in all future PRs where Scala Steward executes the Sbt0_13BuildSyntax rule.

Other notable changes

• The Scalafix CLI has been added to the Docker image.
• Scala Steward checks at start-up if it can run the Scalafix CLI and will print its version.

[codecov]

Codecov Report

Merging #2308 (2519cdf) into master (8599b27) will increase coverage by 1.21%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master    #2308      +/-   ##
==========================================
+ Coverage   78.45%   79.67%   +1.21%     
==========================================
  Files         136      137       +1     
  Lines        2349     2386      +37     
  Branches       58       59       +1     
==========================================
+ Hits         1843     1901      +58     
+ Misses        506      485      -21     

Impacted Files	Coverage Δ
...steward/core/edit/scalafix/ScalafixMigration.scala	88.88% <80.00%> (-11.12%)	⬇️
...la/org/scalasteward/core/application/Context.scala	76.81% <100.00%> (-0.78%)	⬇️
...g/scalasteward/core/application/SelfCheckAlg.scala	100.00% <100.00%> (ø)
...a/org/scalasteward/core/buildtool/sbt/SbtAlg.scala	96.00% <100.00%> (+1.12%)	⬆️
.../scalasteward/core/edit/scalafix/ScalafixCli.scala	100.00% <100.00%> (ø)
.../main/scala/org/scalasteward/core/io/FileAlg.scala	97.05% <100.00%> (+0.08%)	⬆️
...lasteward/core/buildtool/BuildToolDispatcher.scala	100.00% <0.00%> (+23.07%)	⬆️
...ain/scala/org/scalasteward/core/edit/EditAlg.scala	94.44% <0.00%> (+25.00%)	⬆️
... and 1 more

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