[2.x] fix: Allow ++ command to accept project/command patterns not in current state (#8505)

When using '++version project/task', the parser now accepts project/command
patterns (e.g., 'docs/docusaurusPublishGhpages') even if the project doesn't
exist in the current state. This is achieved by adding a fallback parser
that accepts any 'project/command' pattern alongside the combinedParser.

Previously, '++2.12.19 docs/task' would fail with 'Project not found' if
'docs' project wasn't available in the current Scala version, but
'++2.12.19; docs/task' worked. Now both syntaxes work correctly.

The fix is targeted to only accept slash-delimited patterns, avoiding
interference with other parser components like the -v verbose flag.

Fixes #7574

main/src/main/scala/sbt/Cross.scala

@@ -58,7 +58,11 @@ object Cross {
       }
       val spacedVersion = if (spacePresent) version else version & spacedFirst(SwitchCommand)
       val verboseOpt = Parser.opt(token(Space ~> "-v"))
-      val optionalCommand = Parser.opt(token(Space ~> matched(state.combinedParser)))
+      // Accept valid commands, or project/command patterns that may reference projects
+      // not yet available after version switch (fixes #7574)
+      val slashCommand = (NotSpace ~ ('/' ~> any.+)).map { case p ~ c => p + "/" + c.mkString }
+      val commandParser = state.combinedParser | slashCommand
+      val optionalCommand = Parser.opt(token(Space ~> matched(commandParser)))
       val switch1 = (token(Space ~> "-v") ~> (Space ~> version) ~ optionalCommand) map {
         case v ~ command =>
           Switch(v, true, command)