Move bundle loading to own file
No longer support deleted items in bundle’s info.plist

Recording that an item is deleted in the bundle’s info.plist is necessary for TextMate 1.x because it merges all installed bundles, so if the user has a local version, he will still see items from the default version, unless these are indicated to be deleted in the local version.

With 2.0 only delta bundles get merged with a less-local bundle, and when deleting a default item, a delta item is written with the ‘isDeleted’ key set to true.
Avoid member data access when constructing bundles::item_t
Bundle loading no longer done by static member function
Don’t (re)scan when creating fs::node_t

Also make it optional to provide a map pointer (for collecting changes) when rescanning.
Only use cache file if at version 1

Since it’s not unlikely that a user will downgrade from a nightly build, we should be prepared to see cache files using a newer (incompatible) file format.