wasm/linker: symbol resolution improvements

[Luukdegram]

This improves symbol resolution during incremental compilation mode where previously both externs and exports were not being resolved with existing global symbols. This PR ensures during updateDeclExports and getOrPutImport a symbol will be resolved if an existing symbol is found. The symbol resolution follows the same rules as the general symbol resolution for object files.

This also fixes 2 regressions caused by the internpool merge where a merge conflict resulted in undoing previous fixes.