Support import completion for merged declaration exports

[dfreeman]

Fixes #24535

I'm frankly not familiar enough with the codebase to be positive that this is a correct fix, but
this assertion was failing because the result of getAllReExportingModules was empty. The included test failed without this change, and the full suite passes locally with it.

[msftclas]

All CLA requirements met.

[mhegazy]

@Andy-MS can you please review

[ghost]

I think the best fix would be in completions.ts where currently:

const exportedSymbol = skipAlias(symbol.exportSymbol || symbol, checker);

should be:

const exportedSymbol = checker.getMergedSymbol(skipAlias(symbol.exportSymbol || symbol, checker));

Then importFixes has the correct symbol it needs to compare against and doesn't have to work around.

[dfreeman]

@Andy-MS Updated. Thanks for the pointer—that makes much more sense 🙂

[ghost]

Thanks!