Refactor build context for better plugin state sharing

[shuding]

Love the change made in #46666! This PR tries to refactor it by adding an abstraction pluginState object. For the build process, it's just a serializable field that we can attach to the result and resume it as the state at the process beginning. For Webpack plugins, it's an object with all the needed states and their initial values.

The proxy is for lazily initializing these states so they're still global variables, but always initialized after the worker resumes the plugin state.

This way we no longer need to import these plugins from the build worker, and mutate each module export field: Object.assign((flightPluginModule as any)[field], (serializedFlightMaps as any)[field]).

Closes NEXT-758.

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