Add support for :: in --dir to old CLI (bytecodealliance#7416)

In Wasmtime 13 and prior the `--dir` argument was unconditionally used
to open a host dir as the same name and in the guest. In Wasmtime 14+
though this argument is being repurposed with an optional trailing
`::GUEST` to configure the guest directory. This means that
`--dir`-with-remapping behavior is actually unusable without the
environment variable configuration from bytecodealliance#7385 due to it parsing
differently in the old and the new.

This commit updates the situation by adding `::`-parsing to the old CLI
meaning that both the old and the new parse this the same way. This will
break any scripts that open host directories with two colons in their
path, but that seems niche enough we can handle that later.

src/old_cli.rs

@@ -748,7 +748,10 @@ impl RunCommand {
         let mut dirs = Vec::new();
 
         for host in old_dirs {
-            dirs.push((host.clone(), host));
+            let mut parts = host.splitn(2, "::");
+            let host = parts.next().unwrap();
+            let guest = parts.next().unwrap_or(host);
+            dirs.push((host.to_string(), guest.to_string()));
         }
 
         if preview2 {