Make saltutil.sync_all output more meaningful

The default (nested) outputter makes lists of lists look odd. So,
instead of returning a list of lists, return a dict with the keys being
the names of the different custom types, and the values being the lists
of modules of that type that were synced.

Fixes #5964.
Merge pull request #5983 from terminalmage/issue5964

Make saltutil.sync_all output more meaningful