Michael Peyton Jones authored Mar 13, 2019 and Ben Gamari committed Jun 23, 2019

Fixes #16228. The included test case is adapted from the reproduction in
the issue, and fails without this patch.

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We compute an initial visilibity mapping for units based on what is
present in the package databases. To seed this, we compute a set of all
the package configs to add visibilities for.

However, this set was keyed off the unit's *package name*. This is
correct, since we compare packages across databases by version. However,
we would only ever consider a single, most-preferable unit from the
database in which it was found.

The effect of this was that only one of the libraries in a Cabal package
would be added to this initial set. This would cause attempts to use
modules from the omitted libraries to fail, claiming that the package
was hidden (even though `ghc-pkg` would correctly show it as visible).

A solution is to do the selection of the most preferable packages
separately, and then be sure to consider exposing all units in the
...