Fix _is_on_pypi to check response content instead of just catching exceptions (#45933)

PyPIClient.project() returns {'message': 'Not Found'} for non-existent
packages without raising an exception. The _is_on_pypi function only
checked for exceptions, so it incorrectly treated all packages as already
registered on PyPI, preventing name reservation for new packages like
azure-mgmt-appnetwork.

Fix: check for 'info' key in the response, which is only present for
valid PyPI projects.

Author: msyyc

eng/scripts/discover_unpublished_packages.py

@@ -16,8 +16,8 @@
 
 def _is_on_pypi(client, name):
     try:
-        client.project(name)
-        return True
+        result = client.project(name)
+        return "info" in result
     except Exception:
         return False