Extract out xpath to a plugin. (#2661)

tests/shell/mod.rs

@@ -7,7 +7,8 @@ fn plugins_are_declared_with_wix() {
     let actual = nu!(
         cwd: ".", pipeline(
         r#"
-            echo $(open wix/main.wxs --raw | from xml
+            open Cargo.toml | get bin.name | drop | sort-by | wrap cargo | merge {
+                open wix/main.wxs --raw | from xml
                 | get Wix.children.Product.children.0.Directory.children.0
                 | where Directory.attributes.Id == "$(var.PlatformProgramFilesFolder)"
                 | get Directory.children.Directory.children.0 | last
@@ -18,13 +19,9 @@ fn plugins_are_declared_with_wix() {
                 | str substring [_, -4] File.attributes.Name
                 | get File.attributes.Name
                 | sort-by
-                | wrap wix) | merge {
-                    open Cargo.toml |
-                    get bin.name |
-                    drop |
-                    sort-by |
-                    wrap cargo
-                }
+                | wrap wix
+            }
+            | default wix _
             | if $it.wix != $it.cargo { = 1 } { = 0 }
             | math sum
             "#