Ensure licenses.FindThirdPartyDirs produce consistent paths

The patch set in https://crrev.com/c/2267919 refactored the processing
of additional_readme_paths.json files but introduced the change that
such paths will be absolute instead of relative to the root. Other
paths found by walking the directory tree are relative.

This makes such additional paths relative.

Bug: N/A
Change-Id: Ia2b8cfe99d5e704ed423a17f791aea43a94a8d7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2379771
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802667}

tools/licenses.py

@@ -469,10 +469,10 @@ def FilterDirsWithFiles(dirs_list, root):
   return [x for x in dirs_list if ContainsFiles(x, root)]
 
 
-def ProcessAdditionalReadmePathsJson(dirname, third_party_dirs):
+def ProcessAdditionalReadmePathsJson(root, dirname, third_party_dirs):
   """For a given directory, process the additional readme paths, and add to
     third_party_dirs."""
-  additional_paths_file = os.path.join(dirname, ADDITIONAL_PATHS_FILENAME)
+  additional_paths_file = os.path.join(root, dirname, ADDITIONAL_PATHS_FILENAME)
   if os.path.exists(additional_paths_file):
     with open(additional_paths_file) as paths_file:
       extra_paths = json.load(paths_file)
@@ -504,8 +504,7 @@ def FindThirdPartyDirs(prune_paths, root):
         if dirpath not in prune_paths:
           third_party_dirs.add(dirpath)
 
-        additional_paths_dir = os.path.join(root, dirpath)
-        ProcessAdditionalReadmePathsJson(additional_paths_dir, third_party_dirs)
+        ProcessAdditionalReadmePathsJson(root, dirpath, third_party_dirs)
 
       # Don't recurse into any subdirs from here.
       dirs[:] = []
@@ -519,8 +518,7 @@ def FindThirdPartyDirs(prune_paths, root):
   for dir in ADDITIONAL_PATHS:
     if dir not in prune_paths:
       third_party_dirs.add(dir)
-      additional_paths_dir = os.path.join(root, dir)
-      ProcessAdditionalReadmePathsJson(additional_paths_dir, third_party_dirs)
+      ProcessAdditionalReadmePathsJson(root, dir, third_party_dirs)
 
   return third_party_dirs