Fix external renormalization regression (#2153) - #2160

In the shift to code-splitting, there was a regression in f59e310#diff-a1d1f47eb8dcd4b0026545f1e89b1b95.

The issue here has been making the conceptual model of single file builds adapt to the output model of a file structure in code splitting builds, while handling external naming in a way that can be reliably hashed as well as renormalized into the output folder.

In short the fix is to:

1. Adapt the inputBase we're already using for preserveModules to be an input into the preRender function
2. Marking which external modules should be "renormalized" (those that aren't absolute or set by options.paths)
3. Normalizing the external module id relative to the inputBase into a "virtual file space" for code splitting
4. Renormalizing the external module id on chunk write to be relative to the chunk itself, now that we know where it sits int he virtual file space of the build.