fix: xpkg_lua_candidates fallback for default namespace full qualified name

When kDefaultNamespace is "mcpplibs", the canonical .lua candidate for
cmdline becomes "cmdline.lua". But older index clones (and xlings data
dirs with cached index) may still use "mcpplibs.cmdline.lua". Add the
full qualified name as a fallback candidate.

This fixes the release CI failure where the newly built mcpp couldn't
find the index entry during the self-host pack step.

Author: Sunrisepeak

src/pm/compat.cppm

@@ -121,33 +121,43 @@ inline std::vector<std::string> xpkg_lua_candidates(std::string_view ns,
     std::vector<std::string> candidates;
     auto qname = qualified_name(ns, shortName);
 
-    // Canonical: <ns>.<shortName>.lua  (e.g. "compat.mbedtls.lua",
-    //            "mcpplibs.cmdline.lua") — namespace is part of the filename.
-    // For default namespace ("mcpp"), canonical is just "<shortName>.lua".
+    auto fqname = ns.empty() ? std::string(shortName)
+                             : std::format("{}.{}", ns, shortName);
+
+    // Canonical: for default namespace → "<shortName>.lua" (e.g. "cmdline.lua")
+    //            for non-default     → "<ns>.<shortName>.lua" (e.g. "compat.mbedtls.lua")
     if (ns.empty() || ns == mcpp::pm::kDefaultNamespace) {
         candidates.push_back(std::string(shortName) + ".lua");
     } else {
-        candidates.push_back(qname + ".lua");
+        candidates.push_back(fqname + ".lua");
     }
 
     // ── Fallback candidates (COMPAT, remove in 1.0.0) ──────────────
 
-    // Fallback 1: bare short name — covers mcpplibs packages whose
+    // Fallback: full qualified name when it differs from qname
+    // (e.g. default-ns "mcpplibs": qname="cmdline", fqname="mcpplibs.cmdline")
+    auto qnameFile = qname + ".lua";
+    auto fqnameFile = fqname + ".lua";
+    if (fqnameFile != qnameFile &&
+        fqnameFile != candidates.front()) {
+        candidates.push_back(fqnameFile);
+    }
+
+    // Fallback: bare short name — covers mcpplibs packages whose
     // index files are named "<shortName>.lua" without namespace prefix.
     if (!ns.empty() && ns != mcpp::pm::kDefaultNamespace) {
         candidates.push_back(std::string(shortName) + ".lua");
     }
 
-    // Fallback 2: compat.<shortName>.lua — covers compat packages
+    // Fallback: compat.<shortName>.lua — covers compat packages
     // when the caller didn't specify the "compat" namespace.
     if (ns.empty() || ns == mcpp::pm::kDefaultNamespace) {
         candidates.push_back("compat." + std::string(shortName) + ".lua");
     }
 
-    // Fallback 3: compat.<qname>.lua — covers edge cases where the
-    // qualified name itself is used with a compat prefix.
+    // Fallback: compat variants for non-default/non-compat namespaces.
     if (!ns.empty() && ns != mcpp::pm::kDefaultNamespace && ns != "compat") {
-        candidates.push_back("compat." + qname + ".lua");
+        candidates.push_back("compat." + fqname + ".lua");
         candidates.push_back("compat." + std::string(shortName) + ".lua");
     }