input/Open: use OpenLocalInputStream()

Make the "open" method of plugins "file" and "archive" dummy methods
that always fail.  Instead, let InputStream::Open() hard-code access
to these two plugins by using OpenLocalInputStream().  This allows
simplifyin the algorithm for falling back to probing archive plugins.

src/input/plugins/ArchiveInputPlugin.cxx

@@ -71,25 +71,14 @@ OpenArchiveInputStream(Path path, Mutex &mutex, Cond &cond, Error &error)
 	return is;
 }
 
-/**
- * select correct archive plugin to handle the input stream
- * may allow stacking of archive plugins. for example for handling
- * tar.gz a gzip handler opens file (through inputfile stream)
- * then it opens a tar handler and sets gzip inputstream as
- * parent_stream so tar plugin fetches file data from gzip
- * plugin and gzip fetches file from disk
- */
 static InputStream *
-input_archive_open(const char *pathname,
-		   Mutex &mutex, Cond &cond,
-		   Error &error)
+input_archive_open(gcc_unused const char *filename,
+		gcc_unused Mutex &mutex, gcc_unused Cond &cond,
+		gcc_unused Error &error)
 {
-	if (!PathTraitsFS::IsAbsolute(pathname))
-		return nullptr;
+	/* dummy method; use OpenArchiveInputStream() instead */
 
-	/* TODO: the parameter is UTF-8, not filesystem charset */
-	return OpenArchiveInputStream(Path::FromFS(pathname),
-				      mutex, cond, error);
+	return nullptr;
 }
 
 const InputPlugin input_plugin_archive = {

src/input/plugins/FileInputPlugin.cxx

@@ -68,9 +68,8 @@ OpenFileInputStream(Path path,
 {
 	const int fd = OpenFile(path, O_RDONLY|O_BINARY, 0);
 	if (fd < 0) {
-		if (errno != ENOTDIR)
-			error.FormatErrno("Failed to open \"%s\"",
-					  path.c_str());
+		error.FormatErrno("Failed to open \"%s\"",
+				  path.c_str());
 		return nullptr;
 	}
 
@@ -96,17 +95,13 @@ OpenFileInputStream(Path path,
 }
 
 static InputStream *
-input_file_open(const char *filename,
-		Mutex &mutex, Cond &cond,
-		Error &error)
+input_file_open(gcc_unused const char *filename,
+		gcc_unused Mutex &mutex, gcc_unused Cond &cond,
+		gcc_unused Error &error)
 {
-	if (!PathTraitsFS::IsAbsolute(filename))
-		return nullptr;
+	/* dummy method; use OpenFileInputStream() instead */
 
-	/* TODO: the parameter is UTF-8, not filesystem charset */
-	const Path path = Path::FromFS(filename);
-
-	return OpenFileInputStream(path, mutex, cond, error);
+	return nullptr;
 }
 
 bool