Uses of a privately imported name are reported a second time #291

After #278, an import that names a private item reports PrivateSymbol/PrivateType at the import, but doesn't import the item (importing it would be order-dependent; see #289). So each use of the name gets a second error:

import inner::secret;

mod inner {
    fn secret() -> Int { 42 }
}

fn main() -> Int {
    secret()
}

Error: Cannot reference private symbol `secret` in module `inner`
Error: Undeclared symbol `secret`

The second error adds nothing, and there's one for every use. main before #278 also reported the uses (with UnresolvedImport at the import).

One fix: when a named import finds only private items, record the name in the import's scope as blocked, so that resolving it at a use reports nothing more (the way an unresolved name is handled once it's been reported). It must not bind the private item, or #289 comes back. Note that this is only safe once the binding can't change in a later pass, so the blocked name must give way if a public item with that name appears later.

Low priority: the first error is correct and points at the cause.