Problem: No way for plugins to define hooks that would be executed
during plugin lifecycle. Like after install/update or before
update/delete.
Solution: Automatically source scripts defined in plugin manifest file
(if any) after triggering corresponding `PackChanged{,Pre}` events.
During sourcing make some termporary adjustments:
- Current directory is set to plugin's root to simplify execution of
CLI calls like `make build`.
- Plugin's path is ensured to be inside 'runtimepath' to allow using
`require('plugin-module')` inside manifest scripts.
The reason to execute after triggering event is so that there is a
possibility for users to execute code both before the script (exactly
on event) and after the script (more-or-less via `vim.schedule` called
on the event).
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