`librest` in Nixpkgs currently refers to the old 0.8 release. The
current release version is 0.10, which is in Nixpkgs as `librest_1_0`
(since both needed to coexist for some time while dependents were
migrated). Since the old librest version was dependent on insecure
libraries (libsoup 2.4) and has no in-tree dependents, we drop it. We
also rename `librest_1_0` to `librest`, as that's the most
straightforward name, and migrate it to `pkgs/by-name` for cleanliness.
