rust-source, rust-snapshot: simplify as well

* move common stuff from rust-source to rust.inc and from rust-snapshot to rust-target.inc

* 1.34.2 checksums were changed because unified SRC_URI is using tar.xz
  for this version as well (instead of tar.gz used before here)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>

recipes-devtools/rust/rust-source.inc

@@ -0,0 +1,3 @@
+SRC_URI += "https://static.rust-lang.org/dist/rustc-${PV}-src.tar.xz;name=rust"
+
+RUSTSRC = "${WORKDIR}/rustc-${PV}-src"