Cleanup `cargo config` querying

[Veykril]

Right. There is general cleanup potential here. For one, we invoke cargo config three times now for the different things, which is unnecessary, we can just invoke it plain and read out the info from that we want from that output. Secondly, it supports json output (and json-value though I have no idea what that means). We should just use the json output for more robust deserialization.

Originally posted by @Veykril in #20072 (comment)