Prevent emission of (potentially colliding) type aliases in root module for inner types

[jbg]

• Emit aliases in the nearest ancestor module if the parent is not a module, rather than in the root module
• If an alias would refer to itself, don't emit it (this is really a workaround for the fact that names are sometimes not unique enough, e.g. the header in Inner types are not qualified by namespace so may conflict rust-lang/rust-bindgen#2056 produces two items called f_iterator)