Update ImportDeclaration::importedSymbols on-demand

[oxisto]

Currently, we issue ImportDeclaration::updateImportedSymbols for all import declarations in the import resolver and once the type resolver runs. This a) misses out on eventual future declaration updates (as implemented in #2006) and b) is very inefficient, because we only need to update those import declarations that point to a namespace where we change things (in either its scope or child-scopes).

So instead, we should update the symbols on-demand, which probably means that we need a graph edge from the import declaration to the namespace (and back).