merge(#27671): stv/build/list-xblocks

commits
=======
- build: add script to list installed XBlocks

scripts/xblock/list-installed.py

+"""
+Lookup list of installed XBlocks, to aid XBlock developers
+"""
+import pkg_resources
+
+
+def get_without_builtins():
+    """
+    Get all installed XBlocks
+
+    but try to omit built-in XBlocks, else the output is less helpful
+    """
+    xblocks = [
+        entry_point.name
+        for entry_point in pkg_resources.iter_entry_points('xblock.v1')
+        if not entry_point.module_name.startswith('xmodule')
+    ]
+    xblocks = sorted(xblocks)
+    return xblocks
+
+
+def main():
+    """
+    Run the main script
+    """
+    for name in get_without_builtins():
+        print(name)
+
+
+if __name__ == '__main__':
+    main()