resolve: Exclude inaccessible names from single imports

[jseyfried]

If a single import resolves to an inaccessible name in some but not all namespaces, avoid importing the name in the inaccessible namespaces (per rust-lang/rfcs#1560, cc #35120).

Currently, the inaccessible namespaces are imported but cause a privacy error when used.

r? @nrc

[jseyfried]

This is a minor, backwards compatible change that I think would be nice to land outside the item_like_imports feature gate.

Apart from diagnostics, the only observable effect of this PR is to allow, for example:

mod foo {
    pub fn bar() {}
    mod bar {}
}
fn main() {
    use foo::bar; // Before this PR, the inaccessible module `foo::bar` was imported,
    mod bar {} // so this was a duplicate error.
}

[nrc]

@bors: r+

[bors]

📌 Commit 8205691 has been approved by nrc

[alexcrichton]

@bors: rollup

assuming this isn't very platform-specific and green travis means it's good to go whenever