In real EPUB-based VS books, the `index` field of `PageInfo` objects returned by
the `getCurrentPage` and `getPages` methods is undefined. Our mock VS book was
incorrect in this respect.

Correct the mock book to behave like real VS books here, and add a fallback
method to obtain a page index for EPUB-based books by looking up the page in the
list of pages returned by `MosaicBookElement.getPages`.