giomodule: Automatically detect modules on macOS

Piotrek Brzeziński requested to merge thewildtree/glib:macos-gio-modules into main

Makes use of relocatable prefixes, thus allowing modules to be found automatically without needing to set the GIO_EXTRA_MODULES environment variable pointing to them. Tested GStreamer-side with souphttpsrc (which needs a module for HTTPS connections), seems to be working fine.

cc @nirbheek

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