[CT-3336] [Test Performance] Optimize the set_up_plugin_manager() function

[peterallenwebb]

Short description

Profiling shows that the set_up_plugin_manager() function takes up 30% of the runtime on the main thread during integration testing. The hot path is set_up_plugin_manager() -> from_modules() -> iter_modules(). By caching the results we need after the first time iter_modules() is called, nearly all this time can be won back.

Acceptance criteria

• The iter_modules() function is only called once in the above call tree for each test process.
• We have documented a notable improvement in performance in GitHub or via local profile.

Impact to Other Teams

None. They may enjoy the speedier tests, though.

Will backports be required?

No.