support running scalafix on projects with sources at the root

When defining local rules using the Scalafix configuration,
libraryDependencies must be defined, as documented in:
https://scalacenter.github.io/scalafix/docs/developers/local-rules.html.
We could add it by default, but it would unnecessarily clutter the
update task in 99.9% of the plugin use cases.

When projects have a custom directory layout (via scalaSource), users
could not know that Scalafix settings would need to be updated to exclude
sources potentially at the root (sourcesInBase), in order to prevent
compilation errors as Scalafix does not have the Scala libs, causing:

> scala.reflect.internal.MissingRequirementError: object scala in
> compiler mirror not found.

This commit disables sourcesInBase by default to prevent this, as the
local rules have no reason to share code with other default
configurations anyway.

src/main/scala/scalafix/sbt/ScalafixPlugin.scala

@@ -175,7 +175,9 @@ object ScalafixPlugin extends AutoPlugin {
 
   override lazy val projectSettings: Seq[Def.Setting[_]] =
     Seq(Compile, Test).flatMap(c => inConfig(c)(scalafixConfigSettings(c))) ++
-      inConfig(ScalafixConfig)(Defaults.configSettings) ++
+      inConfig(ScalafixConfig)(
+        Defaults.configSettings :+ (sourcesInBase := false)
+      ) ++
       Seq(
         ivyConfigurations += ScalafixConfig,
         scalafixAll := scalafixAllInputTask.evaluated

src/sbt-test/sbt-scalafix/custom-src-directory/app/ProcedureSyntax.scala

@@ -0,0 +1,3 @@
+object ProcedureSyntax {
+  def unit {}
+}

src/sbt-test/sbt-scalafix/custom-src-directory/build.sbt

@@ -0,0 +1,3 @@
+val app = project.settings(
+  scalaSource.in(Compile) := baseDirectory.value
+)

src/sbt-test/sbt-scalafix/custom-src-directory/project/plugins.sbt

@@ -0,0 +1,2 @@
+resolvers += Resolver.sonatypeRepo("public")
+addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % sys.props("plugin.version"))

src/sbt-test/sbt-scalafix/custom-src-directory/test

@@ -0,0 +1,3 @@
+-> app/scalafix --test ProcedureSyntax
+> app/scalafix ProcedureSyntax
+> app/scalafix --test ProcedureSyntax