nativesdk: Package rust sources

This mimics what rustup packages into the toolchain contents. The
presence of the sources for the exact std lib can be useful for the
development and IDE integration.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

recipes-core/packagegroups/packagegroup-rust-cross-canadian.bb

@@ -11,5 +11,6 @@ CARGO="cargo-cross-canadian-${TRANSLATED_TARGET_ARCH}"
 RDEPENDS_${PN} = " \
     ${@all_multilib_tune_values(d, 'RUST')} \
     ${@all_multilib_tune_values(d, 'CARGO')} \
+    rust-cross-canadian-src \
 "