Add sbt.nio.Watch to default imports

I realized writing documentation that it is a pain to not have Watch
available in build.sbt with an import.

sbt/src/main/scala/sbt/Import.scala

@@ -71,6 +71,7 @@ trait Import {
   type RelativeGlob = sbt.nio.file.RelativeGlob
   val RelativeGlob = sbt.nio.file.RelativeGlob
   val RecursiveGlob = sbt.nio.file.RecursiveGlob
+  val Watch = sbt.nio.Watch
 
   // sbt.util
   type AbstractLogger = sbt.util.AbstractLogger

sbt/src/sbt-test/watch/file-input-aggregation/project/Build.scala

@@ -4,7 +4,6 @@ package input.aggregation
 import java.nio.file.Paths
 import sbt.Keys._
 import sbt.internal.DynamicInput
-import sbt.nio.{ file => _, _ }
 import sbt.nio.Keys._
 
 /**

sbt/src/sbt-test/watch/input-parser/project/Build.scala

@@ -6,7 +6,6 @@ import complete.Parser._
 
 import java.io.{ PipedInputStream, PipedOutputStream }
 import Keys._
-import sbt.nio.Watch
 import sbt.nio.Keys._
 
 object Build {

sbt/src/sbt-test/watch/legacy-sources/build.sbt

@@ -1,5 +1,4 @@
 import sbt.legacy.sources.Build._
-import sbt.nio.Watch
 
 Global / watchSources += new sbt.internal.io.Source(baseDirectory.value, "global.txt", NothingFilter, false)
 

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

@@ -1,7 +1,5 @@
 import java.nio.file.Files
 
-import sbt.nio.Watch
-
 import scala.concurrent.duration._
 
 Compile / sourceGenerators += Def.task {

sbt/src/sbt-test/watch/on-change/build.sbt

@@ -1,9 +1,6 @@
 import java.nio.file._
 import java.nio.file.attribute.FileTime
 
-import sbt.nio.Keys._
-import sbt.nio._
-
 import scala.concurrent.duration._
 
 watchTriggeredMessage := { (i, path: Path, c) =>

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

@@ -1,8 +1,6 @@
 import java.nio.file.Files
 import java.nio.file.attribute.FileTime
 
-import sbt.nio.Watch
-
 import scala.concurrent.duration._
 
 val foo = taskKey[Unit]("foo.txt")