Refactor to store dependencies as individual attributes

This also drops support for the transitional `json.depends.array`
configuration value, which has not been necessary since ~2016.

As with tags, dependencies are stored in both a "combined",
comma-separated format (for compatibility) and in an
attribute-per-dependency format (for the future).

src/commands/CmdShow.cpp

@@ -172,7 +172,6 @@ int CmdShow::execute (std::string& output)
     " journal.time.start.annotation"
     " journal.time.stop.annotation"
     " json.array"
-    " json.depends.array"
     " list.all.projects"
     " list.all.tags"
     " locking"