Use SourcePath for reading flake.{nix,lock}

Flakes still reside in the Nix store (so there shouldn't be any change
in behaviour), but they are now accessed via the rootFS
accessor. Since rootFS implements access checks, we no longer have to
worry about flake.{nix,lock} or their parents being symlinks that
escape from the flake.

Extracted from the lazy-trees branch.

src/libexpr/flake/lockfile.cc

@@ -84,8 +84,10 @@ std::shared_ptr<Node> LockFile::findInput(const InputPath & path)
     return doFind(root, path, visited);
 }
 
-LockFile::LockFile(const nlohmann::json & json, const Path & path)
+LockFile::LockFile(std::string_view contents, std::string_view path)
 {
+    auto json = nlohmann::json::parse(contents);
+
     auto version = json.value("version", 0);
     if (version < 5 || version > 7)
         throw Error("lock file '%s' has unsupported version %d", path, version);
@@ -203,12 +205,6 @@ std::pair<std::string, LockFile::KeyMap> LockFile::to_string() const
     return {json.dump(2), std::move(nodeKeys)};
 }
 
-LockFile LockFile::read(const Path & path)
-{
-    if (!pathExists(path)) return LockFile();
-    return LockFile(nlohmann::json::parse(readFile(path)), path);
-}
-
 std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile)
 {
     stream << lockFile.toJSON().first.dump(2);