utils.py: Add a helper for searching upwards for files

i.e. with a given directory and filename, check parent directories until
either a directory with the filename is found, or you reach the root of
the filesystem.

buildstream/utils.py

@@ -1198,3 +1198,17 @@ def _deduplicate(iterable, key=None):
 def _get_link_mtime(path):
     path_stat = os.lstat(path)
     return path_stat.st_mtime
+
+
+# Returns the first directory to contain filename, or an empty string if
+# none found
+#
+def _search_upward_for_file(directory, filename):
+    directory = os.path.abspath(directory)
+    while not os.path.isfile(os.path.join(directory, filename)):
+        parent_dir = os.path.dirname(directory)
+        if directory == parent_dir:
+            return ""
+        directory = parent_dir
+
+    return directory