Mk/Uses/cargo.mk: Fix cargo-crates to not output identical GH_TUPLE entries

For example, in audio/spotifyd some crates have their sources on
GitHub instead of crates.io and share the same repository and commit
in Cargo.lock like

package
name = "librespot"
source = "git+https://github.com/librespot-org/librespot.git#4e3576ba7c6146cf68e1953daeec929d619b26b1"

package
name = "librespot-audio"
source = "git+https://github.com/librespot-org/librespot.git#4e3576ba7c6146cf68e1953daeec929d619b26b1"

Based on this cargo-crates.awk would naively generate multiple
identical entries like

librespot-org:librespot:4e3576ba7c6146cf68e1953daeec929d619b26b1:librespot
librespot-org:librespot:4e3576ba7c6146cf68e1953daeec929d619b26b1:librespotaudio

This adds a lot of extra noise to the port and distinfo and is not
really needed for anything.