home-environment: add home.sessionSearchVariables

This commit introduces `home.sessionSearchVariables` option, that is
created to be a "generic" version of `home.sessionPath` for any
environment variables that is similar to PATH (e.g.: MANPATH). This
allows composition of those variables between multiple modules, avoiding
issues like this one:

https://github.com/nix-community/home-manager/pull/4579/files#r1364374048

This commit also reimplements `home.sessionPath` as terms of
`home.sessionSearchVariables`, to reduce code duplication and show that
the code is correct.

modules/home-environment.nix

@@ -319,6 +319,28 @@ in
       '';
     };
 
+    home.sessionSearchVariables = mkOption {
+      default = { };
+      type = with types; attrsOf (listOf str);
+      example = {
+        MANPATH = [
+          "$HOME/.npm-packages/man"
+          "\${xdg.configHome}/.local/share/man"
+        ];
+      };
+      description = ''
+        Extra directories to add to arbitrary PATH-like environment
+        variables (e.g.: {env}`MANPATH`). The values will be concatenated
+        by `:`.
+
+        These directories are added to the environment variable in a
+        double-quoted context, so expressions like `$HOME` are
+        expanded by the shell. However, since expressions like `~` or
+        `*` are escaped, they will end up in the environment
+        verbatim.
+      '';
+    };
+
     home.sessionVariablesExtra = mkOption {
       type = types.lines;
       default = "";
@@ -555,11 +577,17 @@ in
         export __HM_SESS_VARS_SOURCED=1
 
         ${config.lib.shell.exportAll cfg.sessionVariables}
-      '' + lib.optionalString (cfg.sessionPath != [ ]) ''
-        export PATH="$PATH''${PATH:+:}${concatStringsSep ":" cfg.sessionPath}"
-      '' + cfg.sessionVariablesExtra;
+      '' + concatStringsSep "\n"
+        (mapAttrsToList
+          (env: values: ''
+            export ${env}="''$${env}''${${env}:+:}${concatStringsSep ":" values}"'')
+          cfg.sessionSearchVariables)
+        + cfg.sessionVariablesExtra;
     };
 
+    home.sessionSearchVariables.PATH =
+      mkIf (cfg.sessionPath != [ ]) cfg.sessionPath;
+
     home.packages = [ config.home.sessionVariablesPackage ];
 
     # A dummy entry acting as a boundary between the activation

tests/modules/home-environment/default.nix

@@ -1,4 +1,5 @@
 {
-  home-session-variables = ./session-variables.nix;
   home-session-path = ./session-path.nix;
+  home-session-search-variables = ./session-search-variables.nix;
+  home-session-variables = ./session-variables.nix;
 }

tests/modules/home-environment/session-search-variables.nix

@@ -0,0 +1,15 @@
+{ config, lib, pkgs, ... }:
+
+{
+  imports = [
+    ({ ... }: { config.home.sessionSearchVariables.TEST = [ "foo" ]; })
+    ({ ... }: { config.home.sessionSearchVariables.TEST = [ "bar" "baz" ]; })
+  ];
+
+  nmt.script = ''
+    hmSessVars=home-path/etc/profile.d/hm-session-vars.sh
+    assertFileExists $hmSessVars
+    assertFileContains $hmSessVars \
+      'export TEST="$TEST''${TEST:+:}bar:baz:foo"'
+  '';
+}