DSL: forward-compatible synonyms (WIP)

This makes the following stanzas available in the DSL as
synonyms to existing stanzas, with no change to existing
functionality:
- pkg for `install`
- app for `link`
- suite for `link`
- preflight for `before_install`
- postflight for `before_uninstall`
- uninstall_preflight for `before_uninstall`
- uninstall_postflight for `after_uninstall`

References #4688

This works, but is marked WIP because we are not in a hurry,
and because I intend to add tests.