fix(xlings): build the install_packages args with the JSON library

`[xlings] deps` is manifest input and the arguments were assembled by
formatting the strings into a JSON literal, so a dependency name containing a
quote or a backslash would emit malformed JSON. The failure would then surface
as an xlings parse error naming neither the manifest nor the key that caused
it.

nlohmann::json is already imported in this translation unit (mcpp.libs.json),
so this is `args["targets"] = declaredDeps; args["yes"] = true; args.dump()`
and the escaping stops being something a reader has to verify by eye.

No behaviour change for well-formed names, which is every name in practice --
this is about the failure mode of the one that is not.

Author: speak-agent

src/build/prepare.cppm

@@ -3080,16 +3080,20 @@ prepare_build(bool print_fingerprint,
                     // without pinning it. install_packages resolves the version
                     // itself and reports an ambiguous name with its candidates,
                     // which is the error the author can act on.
-                    std::string targets;
-                    for (auto const& d : declaredDeps) {
-                        if (!targets.empty()) targets += ',';
-                        targets += std::format("\"{}\"", d);
-                    }
+                    // Built with the JSON library rather than by formatting
+                    // the strings in. `deps` is manifest input, so a name
+                    // containing a quote or a backslash would otherwise emit
+                    // malformed JSON and the failure would surface as an
+                    // unrelated xlings parse error naming neither the manifest
+                    // nor the key.
+                    nlohmann::json args;
+                    args["targets"] = declaredDeps;
+                    args["yes"]     = true;
+
                     mcpp::fetcher::InstallProgressHandler progress;
                     auto r = mcpp::xlings::call(
                         mcpp::config::make_xlings_env(**cfg2), "install_packages",
-                        std::format(R"({{"targets":[{}],"yes":true}})", targets),
-                        &progress);
+                        args.dump(), &progress);
                     if (!r) {
                         // Shaped like the toolchain failure: say what failed and
                         // hand back a command the user can run themselves. An