Allow aliases to be used in continuous builds

@japgolly reported in #4695 that aliased commands don't work in watch
anymore. This was because we were extracting the task from the raw
command rather than the aliased command. Since the alias wasn't a valid
key, we weren't able to parse the scoped key. The fix is to find the
aliased value and try that if we fail to parse the original command.

Fixes #4695

main/src/main/scala/sbt/internal/Continuous.scala

@@ -380,12 +380,21 @@ private[sbt] object Continuous extends DeprecatedContinuous {
     // We have to add the <~ Parsers.any.* to ensure that we're able to extract the input key
     // from input tasks.
     val scopedKeyParser: Parser[Seq[ScopedKey[_]]] = Act.aggregatedKeyParser(state) <~ Parsers.any.*
-    Parser.parse(command, scopedKeyParser) match {
-      case Right(scopedKeys: Seq[ScopedKey[_]]) => scopedKeys
-      case Left(e) =>
-        throw new IllegalStateException(s"Error attempting to extract scope from $command: $e.")
-      case _ => Nil: Seq[ScopedKey[_]]
+    @tailrec def impl(current: String): Seq[ScopedKey[_]] = {
+      Parser.parse(current, scopedKeyParser) match {
+        case Right(scopedKeys: Seq[ScopedKey[_]]) => scopedKeys
+        case Left(e) =>
+          val aliases = BasicCommands.allAliases(state)
+          aliases.collectFirst { case (`command`, aliased) => aliased } match {
+            case Some(aliased) => impl(aliased)
+            case _ =>
+              val msg = s"Error attempting to extract scope from $command: $e."
+              throw new IllegalStateException(msg)
+          }
+        case _ => Nil: Seq[ScopedKey[_]]
+      }
     }
+    impl(command)
   }
 
   private def getAllConfigs(

sbt/src/sbt-test/watch/alias/build.sbt

@@ -0,0 +1,6 @@
+val foo = taskKey[Unit]("foo")
+foo := println("foo")
+
+foo / watchOnIteration := { _ => sbt.nio.Watch.CancelWatch }
+addCommandAlias("bar", "foo")
+addCommandAlias("baz", "foo")

sbt/src/sbt-test/watch/alias/test

@@ -0,0 +1,3 @@
+> ~bar
+
+> ~baz