provide a way for users/meson to pass specific pc file

[gdesmott]

While discussing with @ndufresne about gst-plugin-rs's meson integration we figured that meson could detect the presence of an optional dep using pkg-config and cargo/system-deps could end up using another pc file that the one detected by meson, typically if user used the pkg_config_path meson option, leading to confusion.

Maybe meson could communicate somehow to system-deps the full path of the pc files it detected making sure both are using the same one?