fix: use full qualified name for xlings install target

When deps are declared with bare short names (e.g. tinyhttps = "0.2.2"),
the map key is just "tinyhttps" but xlings needs the full qualified name
"mcpplibs.tinyhttps" to find the package in its index. Construct the
fqname from (ns, shortName) for the install target.

Author: Sunrisepeak

src/cli.cppm

@@ -1257,10 +1257,13 @@ prepare_build(bool print_fingerprint,
 
         auto installed = fetcher.install_path(ns, shortName, version);
         if (!installed) {
-            mcpp::ui::info("Downloading", std::format("{} v{}", depName, version));
-            // install() target uses the map key (depName) which xlings
-            // knows how to resolve via its own index lookup.
-            std::vector<std::string> targets{ std::format("{}@{}", depName, version) };
+            // xlings resolves packages by the full qualified name (ns.shortName)
+            // as it appears in the index's name field. Use fqname, not the
+            // map key (which may be a bare short name for default-ns deps).
+            auto fqname = ns.empty() ? shortName
+                : std::format("{}.{}", ns, shortName);
+            mcpp::ui::info("Downloading", std::format("{} v{}", fqname, version));
+            std::vector<std::string> targets{ std::format("{}@{}", fqname, version) };
             CliInstallProgress progress;
             auto r = fetcher.install(targets, &progress);
             if (!r) return std::unexpected(std::format(