Copy forward when registering alias (fixes PaperMC#10827)

When flattening any command or registering an alias to a node with no direct children (such as a redirect), the code now takes into account any potential redirect/fork/forward on the target node. This fixes the issue where, when registering a command that was simply a redirect, only the namespaced literal would work, and not any aliases of the command.

patches/server/0971-Brigadier-based-command-API.patch

@@ -686,10 +686,10 @@ index 0000000000000000000000000000000000000000..1b1642f306771f029e6214a2e2ebebb6
 +}
 diff --git a/src/main/java/io/papermc/paper/command/brigadier/PaperCommands.java b/src/main/java/io/papermc/paper/command/brigadier/PaperCommands.java
 new file mode 100644
-index 0000000000000000000000000000000000000000..da50ca4c6524e4f99ea4de2157d7ef900178d0f1
+index 0000000000000000000000000000000000000000..f6ae741463ed2b64a7b9c66d977b0ef71dd21c8b
 --- /dev/null
 +++ b/src/main/java/io/papermc/paper/command/brigadier/PaperCommands.java
-@@ -0,0 +1,198 @@
+@@ -0,0 +1,199 @@
 +package io.papermc.paper.command.brigadier;
 +
 +import com.google.common.base.Preconditions;
@@ -823,6 +823,7 @@ index 0000000000000000000000000000000000000000..da50ca4c6524e4f99ea4de2157d7ef90
 +        if (redirectTo.getChildren().isEmpty() || hasFlattenRedirectFlag) {
 +            redirect = Commands.literal(aliasLiteral)
 +                .executes(redirectTo.getCommand())
++                .forward(redirectTo.getRedirect(), redirectTo.getRedirectModifier(), redirectTo.isFork())
 +                .requires(redirectTo.getRequirement())
 +                .build();
 +