bugfix: Add output classes directory to classpath for PC

It was there already for bloop and sbt, but not for mill. Because of this, presentation compiler didn't see symbols from workspace.

metals/src/main/scala/scala/meta/internal/metals/MetalsEnrichments.scala

@@ -1015,8 +1015,12 @@ object MetalsEnrichments
         .map(_.stripPrefix(flag))
     }
 
-    def classpath: List[String] =
-      item.getClasspath.asScala.toList
+    def classpath: List[String] = {
+      val outputClasses = item.getClassDirectory
+      val classes = item.getClasspath.asScala.toList
+      if (classes.contains(outputClasses)) classes
+      else outputClasses :: classes
+    }
 
     def jarClasspath: List[AbsolutePath] =
       classpath

tests/slow/src/test/scala/tests/mill/MillServerSuite.scala

@@ -118,4 +118,58 @@ class MillServerSuite
       }
     }
   }
+
+  test(s"presentation-compiler") {
+    def millBspConfig = workspace.resolve(".bsp/mill-bsp.json")
+    writeLayout(
+      s"""
+         |/.mill-version
+         |$supportedBspVersion
+         |/build.sc
+         |import mill._, scalalib._
+         |object foo extends ScalaModule {
+         |  def scalaVersion = "${V.scala213}"
+         |}
+         |/foo/src/Main.scala
+         |package foo
+         |
+         |import foo.A
+         |object Main extends App {
+         |  println(A.msg)
+         |}
+         |
+         |/foo/src/A.scala
+         |package foo
+         |
+         |object A {
+         |  def msg = "Hello"
+         |}
+         |""".stripMargin
+    )
+    for {
+      _ <- server.initialize()
+      _ <- server.initialized()
+      _ = assertNoDiff(
+        client.workspaceMessageRequests,
+        importBuildMessage,
+      )
+      _ = client.messageRequests.clear() // restart
+      _ = assert(!millBspConfig.exists)
+      _ = server.server.buildServerPromise = Promise()
+      _ <- server.executeCommand(ServerCommands.GenerateBspConfig)
+      _ <- server.server.buildServerPromise.future
+      _ = assert(millBspConfig.exists)
+      _ = server.assertBuildServerConnection()
+      _ <- server.didOpen("foo/src/Main.scala")
+      _ <- server.assertHoverAtLine(
+        "foo/src/Main.scala",
+        "println(A.m@@sg)",
+        """|```scala
+           |def msg: String
+           |```
+           |""".stripMargin,
+      )
+
+    } yield {}
+  }
 }