Prevent sbt from compiling .scala files at project root

Previously, creating a .scala file at the root of the dotty repository
would cause sbt to try to compile it when typing `compile`, now it won't
anymore.

Author: smarter

project/Build.scala

@@ -208,7 +208,11 @@ object Build {
     libraryDependencies += "com.novocode" % "junit-interface" % "0.11" % Test,
 
     // enable verbose exception messages for JUnit
-    testOptions in Test += Tests.Argument(TestFrameworks.JUnit, "-a", "-v")
+    testOptions in Test += Tests.Argument(TestFrameworks.JUnit, "-a", "-v"),
+
+    // If someone puts a source file at the root (e.g., for manual testing),
+    // don't pick it up as part of any project.
+    sourcesInBase := false,
   )
 
   // Settings used for projects compiled only with Java