Entrypoint in index.html not rewritten if nested or multiple entrypoints defined

[cbxp]

In case of nested entrypoints, e.g. /_dist_/ui/something.js, baseFile gets only filename 'something' and not 'ui/something' to be replaced properly.

In case of multiple entrypoints, the /_dist_/ prefix is omitted from unhashedEntrypoints and isEntrypoint check always fails.

[KonnorRogers]

@cbxp i actually plan on ripping this feature out. It's brittle and error prone. There's a lot of edge cases and I think it's easier to just tell the user to read from the generated manifest.

#24 addresses it, I just haven't gotten around to making it happen.