Drop support for non-syroot proc macro ABIs
This makes some bigger changes to how we handle the proc-macro-srv things, for one it is now an empty crate if built without the `sysroot-abi` feature, this simplifies some things dropping the need to put the feature cfg in various places. The cli wrapper now actually depends on the server, instead of being part of the server that is just exported, that way we can have a true dummy server that just errors on each request if no sysroot support was specified.
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