Fix plugin reloading for DynamicItems

Since DynamicItems could only ever be loaded by a single
plugin at a time (they are singletons), the code enforcing
this prevented those plugins from being reloaded.

Test Plan: reload the following plugins:

* gravatar-avatar-provider
* websession-flatfile

Bug: issue 2895
Change-Id: I52a07f34d1fa0b756f4cc22d0d83d2ef6906840e
(cherry picked from commit 9a5e78aa73fc4cf1159e2bcca95bf11c1f0b5cb5)