Support inventory macros re-exported by a downstream crate

[dtolnay]

A downstream crate may want to wrap inventory's macros in their own.

gflags::define! {
    /// comma-separated list of languages to offer in the 'lang' menu
    language: &str = "english,french,german";
}

This doesn't work right now because inventory-impl hardcodes inventory as the crate name in generated code.

inventory/impl/src/lib.rs

Line 39 in 1a28fb3

inventory::submit({ #expr });

Fix is blocked on rust-lang/rust#57089.

[dtolnay]

Implemented a workaround in 5a561d3 and dtolnay/gflags@4e188d3.