Block-level DefMap nesting is wrong in inner modules

[jonas-schievink]

#8613 adds a test that demonstrates the issue:

fn f() { // (0)
    mod module {
        struct Struct {}
        fn f() { // (1)
            use self::Struct;
            $0
        }
    }
}

Here, we wrongly store (0) (the block root module) as the module containing (1)'s DefMap, when it should actually be mod module.

Luckily, almost nobody uses modules inside items, so the impact is somewhat limited, but this still points out some issues with how we store inner items, which I do plan to address at some point.