Problem: No way to have full control over how plugin is loaded.
Although `:packadd!` has small side effects (only adds plugin
directory to 'runtimepath'; and maybe its 'after/' subdirectory), it
still has side effects. For example, 'plugin/' directories are still
loaded during startup (as part of `:h load-plugins`).
Solution: Allow function `opts.load` that has full control over how
plugin is loaded.
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