add includePluginResolvers

Fixes #5070

This adds a new setting called `includePluginResolvers` (default `false`).
When set to `true`, it the project will include resolvers from the metabuild.

This allows the build user to declare a resolver in one place (`project/plugins.sbt`) that gets applied to both the metabuild as well as all the subprojects. The scenario comes up when someone distributes a software on their own repo. Ref #4103

main/src/main/scala/sbt/Defaults.scala

@@ -2174,6 +2174,7 @@ object Classpaths {
         moduleConfigurations :== Nil,
         publishTo :== None,
         resolvers :== Vector.empty,
+        includePluginResolvers :== false,
         useJCenter :== false,
         retrievePattern :== Resolver.defaultRetrievePattern,
         transitiveClassifiers :== Seq(SourceClassifier, DocClassifier),
@@ -2246,10 +2247,21 @@ object Classpaths {
       (Def.task {
         val proj = projectResolver.value
         val rs = externalResolvers.value
+        def pluginResolvers: Vector[Resolver] =
+          buildStructure.value
+            .units(thisProjectRef.value.build)
+            .unit
+            .plugins
+            .pluginData
+            .resolvers
+            .getOrElse(Vector.empty)
+        val pr =
+          if (includePluginResolvers.value) pluginResolvers
+          else Vector.empty
         bootResolvers.value match {
           case Some(repos) if overrideBuildResolvers.value => proj +: repos
           case _ =>
-            val base = if (sbtPlugin.value) sbtResolvers.value ++ rs else rs
+            val base = if (sbtPlugin.value) sbtResolvers.value ++ rs ++ pr else rs ++ pr
             (proj +: base).distinct
         }
       }).value,

main/src/main/scala/sbt/Keys.scala

@@ -411,7 +411,8 @@ object Keys {
   val fullResolvers = taskKey[Seq[Resolver]]("Combines the project resolver, default resolvers, and user-defined resolvers.").withRank(CTask)
   val otherResolvers = taskKey[Seq[Resolver]]("Resolvers not included in the main resolver chain, such as those in module configurations.").withRank(CSetting)
   val scalaCompilerBridgeResolvers = taskKey[Seq[Resolver]]("Resolvers used to resolve compiler bridges.").withRank(CSetting)
-  val useJCenter = settingKey[Boolean]("Use JCenter as the default repository.").withRank(BSetting)
+  val includePluginResolvers = settingKey[Boolean]("Include the resolvers from the metabuild.").withRank(CSetting)  
+  val useJCenter = settingKey[Boolean]("Use JCenter as the default repository.").withRank(CSetting)
   val moduleConfigurations = settingKey[Seq[ModuleConfiguration]]("Defines module configurations, which override resolvers on a per-module basis.").withRank(BMinusSetting)
   val retrievePattern = settingKey[String]("Pattern used to retrieve managed dependencies to the current build.").withRank(DSetting)
   val retrieveConfiguration = settingKey[Option[RetrieveConfiguration]]("Configures retrieving dependencies to the current build.").withRank(DSetting)

sbt/src/sbt-test/dependency-management/resolvers-plugin/build.sbt

@@ -0,0 +1,7 @@
+lazy val check = taskKey[Unit]("")
+ThisBuild / includePluginResolvers := true
+
+check := {
+  val rs = fullResolvers.value
+  assert(rs exists (_.name == "bintray-eed3si9n-sbt-plugins"), s"$rs does not include bintray")
+}

sbt/src/sbt-test/dependency-management/resolvers-plugin/project/plugins.sbt

@@ -0,0 +1 @@
+resolvers += Resolver.bintrayRepo("eed3si9n", "sbt-plugins")

sbt/src/sbt-test/dependency-management/resolvers-plugin/test

@@ -0,0 +1 @@
+> check