graphics/libimagequant: convert WRKSRC_SUBDIR to --package

$ make cargo-crates

> /wrkdirs/usr/ports/graphics/libimagequant/work/libimagequant-4.0.5/imagequant-sys/Cargo.lock not found. Trying to generate it...

Updating crates.io index

/usr/bin/awk: can't open file /wrkdirs/usr/ports/graphics/libimagequant/work/libimagequant-4.0.5/imagequant-sys/Cargo.lock
source line number 6

• Error code 2