chore: move all 3p buildRustCrate derivations to //third_party

Profpatsch and me are basically the only users of
depot.users.Profpatsch.writers.rustSimple*. To pull in the odd
dependency we usually use buildRustCrate which is rather convenient.
However we've picked up the bad habit of inlining these in a let
somewhere instead of managing them in a more central location although
there has been an (unsuccesful) attempt at this in
//users/Profpatsch/rust-crates.nix.

This CL moves all buildRustCrate based derivations into
third_party.rust-crates and deletes any duplicate derivations we have
accumulated in the tree.

Change-Id: I8f68b95ebd546708e9af07dca36d72dba9ca8c77
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2769
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: Profpatsch <mail@profpatsch.de>

third_party/rust-crates/OWNERS

@@ -0,0 +1,4 @@
+inherited: true
+owners:
+  - sterni
+  - Profpatsch