fix: Include common in edx-platform packages (#32231)

There may be scenarios, such as third-party plugin test environments,
that import edx-platform as a dependency. For those cases to run smoothly,
`common` needs to be built and exposed as a recognized package.

See https://discuss.openedx.org/t/how-is-the-common-package-installed/10057/2

setup.py

@@ -53,6 +53,7 @@ setup(
     # be reorganized to be a more conventional Python tree.
     packages=[
         "cms",
+        "common",
         "lms",
         "openedx",
         "xmodule",