windows-bindgen filtering and flattening don't just do what they say

[kennykerr]

Summary

• Filtering by namespace doesn't equate to filtering by type if that is something complex like a WinRT class.
• Flattening a namespace does more than just remove the namespace.

Crate manifest

No response

Crate code

windows_bindgen::bindgen([
        "--in",
        "metadata.winmd",
        "--out",
        "src/bindings.rs",
        "--filter",
        "test_constructors", // TODO: this doesn't work when specifying types directly
        "--config",
        "implement",
        "no-bindgen-comment",
        // "flatten", TODO: this does more than just remove the namespace
    ])
    .unwrap();