OrderedImports should create separate groups for different access levels

[allevato]

Now that public/internal/private/etc import is supported, we should have the OrderedImports rule do separate groupings for each access level. Order the groups in descending order of access level (public, package, internal, fileprivate/private), with each group sorted lexicographically as usual.

Other details:

• If a user has both internal import and import, do we unify them by adding/removing internal? We don't currently add or drop internal anywhere else, so we don't have a precedent here.
• We do need to respect the fileprivate/private configuration that's used for other file-scoped declarations.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://126948323