Make bare setting loading order alphabetical

Fixes https://github.com/sbt/sbt/issues/2697
Ref https://twitter.com/not_xuwei_k/status/1230140477959286848

Note that this is could potentially break an existing build that was relying on previous behavior, which seem to return `build.sbt` at the end if you have `a.sbt`, `b.sbt`, `c.sbt`, and `build.sbt`.

main/src/main/scala/sbt/BuildPaths.scala

@@ -8,6 +8,7 @@
 package sbt
 
 import java.io.File
+import java.util.Locale
 import KeyRanks.DSetting
 
 import sbt.io.{ GlobFilter, Path }
@@ -114,7 +115,9 @@ object BuildPaths {
   private[this] def defaultDependencyBase(globalBase: File) = globalBase / "dependency"
   private[this] def defaultGlobalZinc(globalBase: File) = globalBase / "zinc"
 
-  def configurationSources(base: File): Seq[File] = (base * (GlobFilter("*.sbt") - ".sbt")).get
+  def configurationSources(base: File): Seq[File] =
+    (base * (GlobFilter("*.sbt") - ".sbt")).get
+      .sortBy(_.getName.toLowerCase(Locale.ENGLISH))
   def pluginDirectory(definitionBase: File) = definitionBase / PluginsDirectoryName
 
   def evalOutputDirectory(base: File) = outputDirectory(base) / "config-classes"

sbt/src/sbt-test/project/setting-order/a.sbt

@@ -0,0 +1 @@
+Compile / scalacOptions += "a"

sbt/src/sbt-test/project/setting-order/b.sbt

@@ -0,0 +1 @@
+Compile / scalacOptions += "b"

sbt/src/sbt-test/project/setting-order/build.sbt

@@ -0,0 +1,12 @@
+val check = taskKey[Unit]("")
+
+lazy val root = (project in file("."))
+  .settings(
+    Compile / scalacOptions += "multi-project",
+    check := {
+      val xs = (Compile / scalacOptions).value
+      assert(xs.toList == List("multi-project", "a", "b", "bare", "c"), s"$xs")
+    }
+  )
+
+Compile / scalacOptions += "bare"

sbt/src/sbt-test/project/setting-order/c.sbt

@@ -0,0 +1 @@
+Compile / scalacOptions += "c"

sbt/src/sbt-test/project/setting-order/test

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