ui: Only import plugins with an index.ts

- Currently if a plugin dir is left empty, perfetto will try to import
it and the build will fail.
- This CL adds a check to ensure plugin dirs actually contain an
index.ts before adding it to the import list.

Change-Id: I262a2cd10ccadcef87179cc18ca80e39e10bebba

tools/gen_ui_imports

@@ -49,9 +49,15 @@ def to_camel_case(s):
   return first + ''.join(x.title() for x in rest)
 
 
+def is_plugin_dir(dir):
+  # Ensure plugins contain a file called index.ts. This avoids the issue empty
+  # dirs are detected as plugins.
+  return os.path.isdir(dir) and os.path.exists(os.path.join(dir, 'index.ts'))
+
+
 def gen_imports(input_dir, output_path):
   paths = [os.path.join(input_dir, p) for p in os.listdir(input_dir)]
-  paths = [p for p in paths if os.path.isdir(p)]
+  paths = [p for p in paths if is_plugin_dir(p)]
   paths.sort()
 
   output_dir = os.path.dirname(output_path)
@@ -60,6 +66,7 @@ def gen_imports(input_dir, output_path):
   imports = []
   registrations = []
   for path in paths:
+    # Get out if 
     rel_path = os.path.relpath(path, output_dir)
     snake_name = os.path.basename(path)
     camel_name = to_camel_case(snake_name)