collect, but don't assert, types from all modules

[mndrix]

We currently ignore structured comments from modules that don't explicitly use mavis. Instead, we should process all structured comments and add the details to our database but don't perform term_expansion on them.

This will facilitate type inference and mode inference later.

[mndrix]

Now that library(pldoc) collects comments for us, we should instead facilitate type inference by creating a single predicate which gives access to normalized mode information based on a predicate indicator or predicate head.

This is similar to pldoc_modes:mode/2 which accepts a module-qualified head term and populates it with mode information and determinism. The only difference between that predicate and ours is that we retain the name of each argument.