construct helm sources with macros rather than using alists

[dankessler]

see this PR for spacemacs for an entirely analogous fix.

In brief, helm supports constructing sources directly as alists, but discourages it. Building the sources using macros fully supports the helm feature set, including space-delimited search patterns, which helm-mode-manager doesn't currently support (e.g., entering org mode fails to match org-mode).