Fix issue with duplicate reexported definitions (T23616)

When a class method was reexported, it's default methods were also
showing up in the generated html page.

The simplest and most non-invasive fix is to not look for the default
method if we are just exporting the class method.. because the backends
are just showing default methods when the whole class is exported.

In general it would be worthwhile to rewrite this bit of code I think as
the logic and what gets included is split over `lookupDocs` and
`availExportDecl` it would be clearer to combine the two. The result of
lookupDocs is always just passed to availExportDecl so it seems simpler
and more obvious to just write the function directly.