DUB supports "customCachePaths" for providing read-only package paths

With this release DUB allows defining additional paths that contain packages in subfolders with the pattern "(name)-(version)/(name)/" by defining a "customCachePaths" field in `/etc/dub/settings.json` or `~/.dub/settings.json.`

"customCachePaths" can be used to provide prebuilt DUB libraries (e.g. for distribution package maintainers).