API inconsistency, manifest absolute file path: '/' character prefix when no "root" content folder (because of Package->BasePath())

[danielweck]

Manifest Items absolute paths are returned with prefix '/' when the EPUB OPF is not located within the typical subfolder (OEBPS, EPUB, etc.)
So, instead of say: "OEBPS/html/chapter1.xhtml", it returns "/html/chapter1.xhtml" !!
(note the '/' root path discrepancy)

This is due to Package->BasePath() returning "/" in one case, but "OEBPS/" instead of "/OEBPS/" in the other case.
https://github.com/readium/readium-sdk/blob/develop/ePub3/ePub/package.h#L124
https://github.com/readium/readium-sdk/blob/develop/ePub3/ePub/package.cpp#L111

See related Media Overlays fix:
4ad3e73

See related FontObfuscation fix:
1a43b1e

[danielweck]

@rkwright SmokeTestFXL obfuscated fonts fail because of this issue (encryption info cannot be matched against absolute paths with '/' prefix). Working on this.