Ambiguous names in imports aren't refused #179

Three ways a name means two things, all of which build:

An import shadowing a local declaration:

use types::S       // types.ids also declares S
struct S {
    a: u8
}

Two glob imports exporting the same name, then using it:

use one::*         // declares T
use two::*         // also declares T
struct R {
    t: T
}

A dependency with the same name as one of the package's own schemas:

dependencies = {
    api = { path = "dep" }     # the package also has src/api.ids
}

use api::S

This last one builds, and logs an internal warning, Symbol 'S' not found, leaving unclear which api::S was used.

Fix: an error for each ambiguity, naming both sources: an import that clashes with a local name; a name two glob imports provide, where it's used; a dependency key equal to an own schema's name.

Found on core master (a8aee68).