rename logshipper -> logtap

- it matches the executable name
- it doesn't ship anything anyway, it's just plumbing

pkgs/logtap/default.nix

@@ -0,0 +1,8 @@
+{
+  stdenv,
+}:
+stdenv.mkDerivation {
+  name = "logtap";
+  makeFlags = [ "PREFIX=${placeholder "out"}" ];
+  src = ./.;
+}