[M6] resolver: pkg-config probe + codegen for PkgConfig

src/resolver/resolver.cppm

@@ -77,6 +77,22 @@ auto nix_cmake_scan(const std::filesystem::path& store_path,
                     const std::string& package_name)
     -> util::Result<NixCmakeCandidate>;
 
+// A pkg-config-shaped recipe: the package ships a `.pc` file rather
+// than a CMake config. Consumed via `find_package(PkgConfig REQUIRED)`
+// + `pkg_check_modules(<NAME> REQUIRED IMPORTED_TARGET <pc_module>)`,
+// linking against the generated `PkgConfig::<NAME>` target.
+struct PcCandidate {
+    std::string pc_module;             // file stem, e.g. "sqlite3"
+    std::filesystem::path pc_file;     // path to the .pc on disk
+};
+
+// Walks <store_path>/lib/pkgconfig/*.pc and picks the best match for
+// `package_name`. Returns ResolutionUnknownPackage when no `.pc` file
+// is present or none scores acceptably.
+auto pc_scan(const std::filesystem::path& store_path,
+             const std::string& package_name)
+    -> util::Result<PcCandidate>;
+
 // Output of a conan-center-index recipe scrape.
 struct ConanRecipe {
     std::string find_package; // e.g. "fmt CONFIG REQUIRED"