Monitor project build sources

In sbt 1.3.0, we only monitor build sources in the root project
directory and the root project meta build directory. This commit adds
these inputs for each project.

Fixes https://github.com/sbt/sbt/issues/5061.

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

@@ -371,13 +371,16 @@ object Defaults extends BuildCommon {
       }).value
   )
 
-  def paths = Seq(
+  // Appended to JvmPlugin.projectSettings
+  def paths: Seq[Setting[_]] = Seq(
     baseDirectory := thisProject.value.base,
     target := baseDirectory.value / "target",
     historyPath := (historyPath or target(t => Option(t / ".history"))).value,
     sourceDirectory := baseDirectory.value / "src",
     sourceManaged := crossTarget.value / "src_managed",
-    resourceManaged := crossTarget.value / "resource_managed"
+    resourceManaged := crossTarget.value / "resource_managed",
+    // Adds subproject build.sbt files to the global list of build files to monitor
+    Scope.Global / checkBuildSources / fileInputs += baseDirectory.value.toGlob / "*.sbt"
   )
 
   lazy val configPaths = sourceConfigPaths ++ resourceConfigPaths ++ outputConfigPaths

sbt/src/sbt-test/nio/reload/build.sbt

@@ -11,3 +11,5 @@ exists := {
 }
 
 Global / onChangedBuildSource := ReloadOnSourceChanges
+
+val sub = project

sbt/src/sbt-test/nio/reload/changes/broken.sbt

@@ -8,3 +8,5 @@ exists := {
 }
 
 Global / onChangedBuildSource := ReloadOnSourceChanges
+
+val sub = project

sbt/src/sbt-test/nio/reload/changes/sub.sbt

@@ -0,0 +1 @@
+libraryDependencies += "org.scala-sbt" % "sbt" % "1.3.0"

sbt/src/sbt-test/nio/reload/changes/working.sbt

@@ -11,3 +11,5 @@ exists := {
 }
 
 Global / onChangedBuildSource := ReloadOnSourceChanges
+
+val sub = project

sbt/src/sbt-test/nio/reload/sub/src/main/scala/Test.scala

@@ -0,0 +1,3 @@
+object Test {
+  val x = sbt.Keys
+}

sbt/src/sbt-test/nio/reload/test

@@ -17,3 +17,9 @@ $ copy-file changes/working.sbt build.sbt
 > foo foo; reload
 
 > exists foo
+
+-> compile
+
+$ copy-file changes/sub.sbt sub/build.sbt
+
+> compile