Feature/r autocomplete help

This modifies the `get_help` RPC call to be more restrictive on when help is shown. It should help to eliminate 'spurious' help from e.g. variables named identically to functions, and so on.

Currently, we support help for functions, function arguments, and packages -- in the future, we could imagine adding support for data sets as well.

Note that I've changed the `pkgName` parameter name to `source`, which is now more appropriate given that `QualifiedName`s can now come from many 'sources' -- that string combined with the type should provide enough information to resolve where a particular qualified name is from.