Move logic to execute HookLoadFile for signatures into rule matcher code.

This (1) fixes an issue where signature files supplied on the command
line wouldn't pass through the hooks, and (2) prepares for allowing
hooks to supply the content of a signature file directly.

src/ScannedFile.h

@@ -3,6 +3,7 @@
 #pragma once
 
 #include <list>
+#include <optional>
 #include <string>
 #include <vector>
 
@@ -34,6 +35,16 @@ public:
 	};
 
 extern std::list<ScannedFile> files_scanned;
-extern std::vector<std::string> sig_files;
+
+struct SignatureFile
+	{
+	std::string file;
+	std::optional<std::string> full_path;
+
+	SignatureFile(std::string file);
+	SignatureFile(std::string file, std::string full_path);
+	};
+
+extern std::vector<SignatureFile> sig_files;
 
 	} // namespace zeek::detail