Disregard transient cache in perflab_query_plugin_info() when a plugin is absent

[b1ink0]

Summary

Fixes #1617

Relevant technical choices

When a new feature plugin is published, it may not yet exist in the transient. Therefore, whenever one of the expected feature plugin is absent, we should act as if the transient was not set in the first place so that we can obtain the latest plugin info right away.

Logic for this to work:

• If the plugin slug is not found in the transient then instead of returning a error we proceeded to fetch information from API.
• If the plugin slug is also not found in the API it is also stored in transient but with key as plugin slug and value as false.
• If the plugin slug is found in the transient but if its value is equal to false raise the error.

This logic will make sure that the multiple request are not made for the plugin which is absent.

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