beats 7.x: refactor

Replace `recursiveUpdate` with `//` and explicit `meta`/`passthru`
merging so that `builtins.unsafeGetAttrPos` can find `meta.description`
at each call site, making the `pos = __curPos` workaround unnecessary.

Also adopt the `finalAttrs` pattern and use `tag` instead of `rev` in
`fetchFromGitHub`.

pkgs/misc/logging/beats/7.x.nix

@@ -13,14 +13,15 @@ let
   beat =
     package: extraArgs:
     buildGoModule (
-      lib.attrsets.recursiveUpdate rec {
+      finalAttrs:
+      {
         pname = package;
         version = elk7Version;
 
         src = fetchFromGitHub {
           owner = "elastic";
           repo = "beats";
-          rev = "v${version}";
+          tag = "v${finalAttrs.version}";
           hash = "sha256-TzcKB1hIHe1LNZ59GcvR527yvYqPKNXPIhpWH2vyMTY=";
         };
 
@@ -36,17 +37,19 @@ let
             dfithian
           ];
           platforms = lib.platforms.linux;
-        };
-      } extraArgs
+        }
+        // (extraArgs.meta or { });
+      }
+      // removeAttrs extraArgs [
+        "meta"
+      ]
     );
 in
 rec {
   auditbeat7 = beat "auditbeat" {
-    pos = __curPos;
     meta.description = "Lightweight shipper for audit data";
   };
   filebeat7 = beat "filebeat" {
-    pos = __curPos;
     meta.description = "Lightweight shipper for logfiles";
     buildInputs = [ systemd ];
     tags = [ "withjournald" ];
@@ -55,11 +58,9 @@ rec {
     '';
   };
   heartbeat7 = beat "heartbeat" {
-    pos = __curPos;
     meta.description = "Lightweight shipper for uptime monitoring";
   };
   metricbeat7 = beat "metricbeat" {
-    pos = __curPos;
     meta.description = "Lightweight shipper for metrics";
     passthru.tests = lib.optionalAttrs config.allowUnfree (
       assert metricbeat7.drvPath == nixosTests.elk.unfree.ELK-7.elkPackages.metricbeat.drvPath;
@@ -70,7 +71,6 @@ rec {
   };
   packetbeat7 = beat "packetbeat" {
     buildInputs = [ libpcap ];
-    pos = __curPos;
     meta.description = "Network packet analyzer that ships data to Elasticsearch";
     meta.longDescription = ''
       Packetbeat is an open source network packet analyzer that ships the