modules/lsp: allow servers to install multiple packages

Adds two internal per-server options: `packages.prefix` & `packages.suffix`.

These options allow the server module to install multiple packages, and
control which end up being prefixed or suffixed on the PATH.

This simplifies the propagating code in `modules/lsp/servers/default.nix`,
which can now zip up the enabled server `packages` attrs.

modules/lsp/servers/default.nix

@@ -153,9 +153,8 @@ in
       ];
 
       packages = lib.pipe enabledServers [
-        (builtins.filter (server: server ? package))
+        (builtins.catAttrs "packages")
-        (builtins.groupBy (server: if server.packageFallback then "suffix" else "prefix"))
+        (builtins.zipAttrsWith (_: builtins.concatLists))
-        (builtins.mapAttrs (_: builtins.catAttrs "package"))
       ];
     in
     {

modules/lsp/servers/server.nix

@@ -64,6 +64,22 @@ in
       '';
     };
 
+    packages.prefix = lib.mkOption {
+      type = types.listOf types.package;
+      description = "Packages to prefix onto the PATH.";
+      default = [ ];
+      visible = false;
+      internal = true;
+    };
+
+    packages.suffix = lib.mkOption {
+      type = types.listOf types.package;
+      description = "Packages to suffix onto the PATH.";
+      default = [ ];
+      visible = false;
+      internal = true;
+    };
+
     config = lib.mkOption {
       type = with types; attrsOf anything;
       description = ''
@@ -88,6 +104,14 @@ in
     };
   };
 
+  config = {
+    packages = lib.mkIf (config.package != null) {
+      ${if config.packageFallback then "suffix" else "prefix"} = [
+        config.package
+      ];
+    };
+  };
+
   imports = [
     ./server-renames.nix
   ]