There was a race in (*App).SyncPlugins where if the same plugin
existed in availablePlugins and pluginSignaturePathMap, then
we would try to add/remove at the same time.
This would lead to a possible removal / addition of a plugin directory
or even unable to remove a directory because it was already in use.
We fix this by first finishing the removal of availablePlugins
before syncing it with the file store.
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