Build NOTE: add utility functions to avoid using unexported objects

[jacobvjk]

copies: RMI-PACTA/pacta.aggregate.loanbook.plots#100

Unexported objects imported by ':::' calls:
    ‘r2dii.plot:::abort_if_missing_names’
    ‘r2dii.plot:::abort_if_unknown_values’
    See the note in ?`:::` about the use of this operator.

we decided to add the relevant functions to this package rather than exporting them in r2dii.plot

[jacobvjk]

This actually relates to prep_*() and plot_*() functions only, so it should be tackled in pacta.multi.loanbook.plot once the relevant functions are migrated

[jacobvjk]

closed via #16