Allow missing package.metadata.bundle section

[madsmtm]

This allows running cargo bundle on a project which has not been directly configured for it, which can be quite nice when using the iOS support to run under Mac Catalyst.

Example of such usage:

cargo +nightly build --target aarch64-apple-ios-macabi -Zbuild-std
CARGO_BUNDLE_SKIP_BUILD=1 cargo bundle --target aarch64-apple-ios-macabi --format ios
target/aarch64-apple-ios-macabi/debug/bundle/ios/my_app.app/my_app

This pairs especially well with #182.