slice2swift Does not Correctly Generate Links to Symbols in Nested Modules

[InsertCreativityHere]

In Swift, packages are flat, meaning things can only live 1 package deep, there is no notion of "nested packages".
To fix this, any Slice constructs which are within nested modules will have the extra modules rolled into their names. Ex:

["swift:module:Ice:MX]
module IceMX
{
    struct MetricsFailures { ... }
}

will generate a struct named MXMetricsFailures which lives inside the Ice package.

This approach really screws with doc-comments.
Let's say we now do {@link MetricsFailures}. All we see is this string. There's no collision with reserved identifiers or keywords, so we don't change the string, and generate a Swift link to it: ``MetricsFailures``. However, nothing with that name exists. Since it should be mapped to MXMetricsFailures.

[InsertCreativityHere]

I encountered this problem in slice2swift, but I bet that this problem also exists anywhere where users use metadata to map module names to something else. And will become a huge headache once we add xxx:identifier metadata.

So the fix isn't specific to slice2swift, but is instead to perform some kind of type-lookup on any @links, instead of just passing the raw string along. This would also have the added benefit that we could issue warnings for dead links.