os: let `os.dir("/xyz")` return "/" (fixes compiling .v files in /)

vlib/os/os.v

@@ -170,6 +170,9 @@ pub fn dir(path string) string {
 		return '.'
 	}
 	pos := path.last_index(path_separator) or { return '.' }
+	if pos == 0 && path_separator == '/' {
+		return '/'
+	}
 	return path[..pos]
 }
 

vlib/os/os_test.v

@@ -534,6 +534,8 @@ fn test_dir() {
 		assert os.dir('C:\\a\\b\\c') == 'C:\\a\\b'
 		assert os.dir('C:\\a\\b\\') == 'C:\\a\\b'
 	} $else {
+		assert os.dir('/') == '/'
+		assert os.dir('/abc') == '/'
 		assert os.dir('/var/tmp/foo') == '/var/tmp'
 		assert os.dir('/var/tmp/') == '/var/tmp'
 	}