[bldr-build] Add explicit function visibility/external API hints.
[bldr-build] Add explicit function visibility/external API hints.

This change changes the names of some internally-facing functions in
order to signal that external users (i.e. Plan authors) should not call
these functions or rely on their current behavior over the long term. A
function which is prefix with a single underscore (`_`) is API/private
visibility.

be-plans/be,be-plans/be,be-plans/be,be-plans/be