[172848427] Rework plug-in initialization to support a new plug-in layout.#1259

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cwakamo:eng/PR-172848427-restructure-for-iPadOS-Swift-Playground-app-store-ingestioncwakamo/swift-build:eng/PR-172848427-restructure-for-iPadOS-Swift-Playground-app-store-ingestionCopy head branch name to clipboard