Fix multi-repl when only building some internal library targets

When combining together --dependency and --promised-dependency flags, we
were using `Map.union` in the wrong place. If you had a dependency
and promised-dependency from the same package (ie when using an internal
library) then the promised dependency wouldn't be taken into account.

The fix is straightforward, don't use `Map.union`. First create a list
of everything and then create a map using `fromListWith`.

Fixes #10775

(cherry picked from commit c2cdfc01)