ref(nextjs): Simplify rollupize in proxy loader

[lobsterkatie]

This makes a few small changes to the rollupize function used by the nextjs SDK's proxy loader:

• Consolidate error handling in the proxy loader, rather than handling errors inside of rollupize and then handling an undefined rollupize return value in the proxy loader.
• Slightly simplify explanation of makeAbsoluteExternalsRelative
• Remove type hack in getRollupInputOptions which seems no longer to be needed. (It was never clear why it was necessary in the first place, and TS no longer seems mad when it's removed.)
• Rename resourcePath to userModulePath for clarity

The latter two changes are drawn from #5960, to preserve them even though it is being reverted.